diff --git a/include/picrin.h b/include/picrin.h
index fa84f59b..d194de1f 100644
--- a/include/picrin.h
+++ b/include/picrin.h
@@ -105,8 +105,8 @@ typedef struct {
   struct pic_jmpbuf *try_jmps;
 
   struct pic_heap *heap;
-  struct pic_object *arena[PIC_ARENA_SIZE];
-  int arena_idx;
+  struct pic_object **arena;
+  size_t arena_size, arena_idx;
 
   char *native_stack_start;
 } pic_state;
@@ -123,8 +123,8 @@ void pic_free(pic_state *, void *);
 
 void pic_gc_run(pic_state *);
 pic_value pic_gc_protect(pic_state *, pic_value);
-int pic_gc_arena_preserve(pic_state *);
-void pic_gc_arena_restore(pic_state *, int);
+size_t pic_gc_arena_preserve(pic_state *);
+void pic_gc_arena_restore(pic_state *, size_t);
 
 pic_state *pic_open(int argc, char *argv[], char **envp);
 void pic_close(pic_state *);
diff --git a/include/picrin/cont.h b/include/picrin/cont.h
index a4b8f32a..496454fd 100644
--- a/include/picrin/cont.h
+++ b/include/picrin/cont.h
@@ -26,7 +26,8 @@ struct pic_cont {
 
   pic_code *ip;
 
-  struct pic_object *arena[PIC_ARENA_SIZE];
+  struct pic_object **arena;
+  size_t arena_size;
   int arena_idx;
 
   pic_value results;
diff --git a/src/codegen.c b/src/codegen.c
index ef9bd216..2d052739 100644
--- a/src/codegen.c
+++ b/src/codegen.c
@@ -266,7 +266,7 @@ static pic_value
 analyze(analyze_state *state, pic_value obj, bool tailpos)
 {
   pic_state *pic = state->pic;
-  int ai = pic_gc_arena_preserve(pic);
+  size_t ai = pic_gc_arena_preserve(pic);
   pic_value res;
   pic_sym tag;
 
@@ -1438,7 +1438,7 @@ struct pic_proc *
 pic_compile(pic_state *pic, pic_value obj)
 {
   struct pic_irep *irep;
-  int ai = pic_gc_arena_preserve(pic);
+  size_t ai = pic_gc_arena_preserve(pic);
 
 #if DEBUG
   fprintf(stdout, "ai = %d\n", pic_gc_arena_preserve(pic));
diff --git a/src/cont.c b/src/cont.c
index e22bcfef..f84e55c7 100644
--- a/src/cont.c
+++ b/src/cont.c
@@ -139,7 +139,9 @@ save_cont(pic_state *pic, struct pic_cont **c)
   cont->ip = pic->ip;
 
   cont->arena_idx = pic->arena_idx;
-  memcpy(cont->arena, pic->arena, sizeof(struct pic_object *) * PIC_ARENA_SIZE);
+  cont->arena_size = pic->arena_size;
+  cont->arena = (struct pic_object **)pic_alloc(pic, sizeof(struct pic_object *) * pic->arena_size);
+  memcpy(cont->arena, pic->arena, sizeof(struct pic_object *) * pic->arena_size);
 
   cont->results = pic_undef_value();
 }
@@ -182,7 +184,9 @@ restore_cont(pic_state *pic, struct pic_cont *cont)
 
   pic->ip = cont->ip;
 
-  memcpy(pic->arena, cont->arena, sizeof(struct pic_object *) * PIC_ARENA_SIZE);
+  pic->arena = (struct pic_object **)pic_realloc(pic, pic->arena, sizeof(struct pic_object *) * cont->arena_size);
+  memcpy(pic->arena, cont->arena, sizeof(struct pic_object *) * cont->arena_size);
+  pic->arena_size = cont->arena_size;
   pic->arena_idx = cont->arena_idx;
 
   memcpy(cont->stk_pos, cont->stk_ptr, cont->stk_len);
diff --git a/src/debug.c b/src/debug.c
index 7ce89fc0..f59a4125 100644
--- a/src/debug.c
+++ b/src/debug.c
@@ -10,7 +10,7 @@
 pic_str *
 pic_get_backtrace(pic_state *pic)
 {
-  int ai = pic_gc_arena_preserve(pic);
+  size_t ai = pic_gc_arena_preserve(pic);
   pic_callinfo *ci;
   pic_str *trace;
 
@@ -38,7 +38,7 @@ pic_get_backtrace(pic_state *pic)
 void
 pic_print_backtrace(pic_state *pic, struct pic_error *e)
 {
-  int ai = pic_gc_arena_preserve(pic);
+  size_t ai = pic_gc_arena_preserve(pic);
   pic_str *trace;
 
   assert(pic->err != NULL);
diff --git a/src/gc.c b/src/gc.c
index fd48833e..2821be35 100644
--- a/src/gc.c
+++ b/src/gc.c
@@ -179,8 +179,9 @@ pic_free(pic_state *pic, void *ptr)
 static void
 gc_protect(pic_state *pic, struct pic_object *obj)
 {
-  if (pic->arena_idx >= PIC_ARENA_SIZE) {
-    pic_abort(pic, "arena overflow");
+  if (pic->arena_idx >= pic->arena_size) {
+    pic->arena_size = pic->arena_size * 2 + 1;
+    pic->arena = pic_realloc(pic, pic->arena, sizeof(struct pic_object *) * pic->arena_size);
   }
   pic->arena[pic->arena_idx++] = obj;
 }
@@ -200,14 +201,14 @@ pic_gc_protect(pic_state *pic, pic_value v)
   return v;
 }
 
-int
+size_t
 pic_gc_arena_preserve(pic_state *pic)
 {
   return pic->arena_idx;
 }
 
 void
-pic_gc_arena_restore(pic_state *pic, int state)
+pic_gc_arena_restore(pic_state *pic, size_t state)
 {
   pic->arena_idx = state;
 }
@@ -618,6 +619,7 @@ gc_finalize_object(pic_state *pic, struct pic_object *obj)
     pic_free(pic, cont->stk_ptr);
     pic_free(pic, cont->st_ptr);
     pic_free(pic, cont->ci_ptr);
+    pic_free(pic, cont->arena);
     PIC_BLK_DECREF(pic, cont->blk);
     break;
   }
diff --git a/src/init.c b/src/init.c
index 8bbc3229..26050194 100644
--- a/src/init.c
+++ b/src/init.c
@@ -60,7 +60,7 @@ pic_features(pic_state *pic)
 void
 pic_init_core(pic_state *pic)
 {
-  int ai = pic_gc_arena_preserve(pic);
+  size_t ai = pic_gc_arena_preserve(pic);
 
   pic_deflibrary ("(scheme base)") {
 
diff --git a/src/load.c b/src/load.c
index ec95b4dc..b1fcf39a 100644
--- a/src/load.c
+++ b/src/load.c
@@ -8,7 +8,7 @@
 pic_value
 pic_load_cstr(pic_state *pic, const char *src)
 {
-  int ai;
+  size_t ai;
   pic_value v, exprs;
   struct pic_proc *proc;
 
@@ -37,7 +37,7 @@ pic_value
 pic_load(pic_state *pic, const char *fn)
 {
   FILE *file;
-  int ai;
+  size_t ai;
   pic_value v, exprs;
   struct pic_proc *proc;
 
diff --git a/src/macro.c b/src/macro.c
index b4a33a5a..2790efb0 100644
--- a/src/macro.c
+++ b/src/macro.c
@@ -150,7 +150,7 @@ static pic_value macroexpand_node(pic_state *, pic_value, struct pic_senv *, pic
 static pic_value
 macroexpand(pic_state *pic, pic_value expr, struct pic_senv *senv, pic_value assoc_box)
 {
-  int ai = pic_gc_arena_preserve(pic);
+  size_t ai = pic_gc_arena_preserve(pic);
   pic_value v;
 
   v = macroexpand_node(pic, expr, senv, assoc_box);
@@ -196,7 +196,7 @@ push_scope(pic_state *pic, pic_value formals, struct pic_senv *up, pic_value ass
 static pic_value
 macroexpand_list(pic_state *pic, pic_value list, struct pic_senv *senv, pic_value assoc_box)
 {
-  int ai = pic_gc_arena_preserve(pic);
+  size_t ai = pic_gc_arena_preserve(pic);
   pic_value v, vs;
 
   /* macroexpand in order */
@@ -271,7 +271,7 @@ macroexpand_deflibrary(pic_state *pic, pic_value expr)
     pic_in_library(pic, pic_cadr(pic, expr));
 
     pic_for_each (v, pic_cddr(pic, expr)) {
-      int ai = pic_gc_arena_preserve(pic);
+      size_t ai = pic_gc_arena_preserve(pic);
 
       pic_eval(pic, v);
 
diff --git a/src/number.c b/src/number.c
index a73eb785..fb9a4c1f 100644
--- a/src/number.c
+++ b/src/number.c
@@ -734,7 +734,7 @@ pic_number_string_to_number(pic_state *pic)
 void
 pic_init_number(pic_state *pic)
 {
-  int ai = pic_gc_arena_preserve(pic);
+  size_t ai = pic_gc_arena_preserve(pic);
 
   pic_defun(pic, "number?", pic_number_real_p);
   pic_defun(pic, "complex?", pic_number_real_p);
diff --git a/src/pair.c b/src/pair.c
index 37d5ff7f..6fe316c1 100644
--- a/src/pair.c
+++ b/src/pair.c
@@ -84,7 +84,7 @@ pic_list1(pic_state *pic, pic_value obj1)
 pic_value
 pic_list2(pic_state *pic, pic_value obj1, pic_value obj2)
 {
-  int ai = pic_gc_arena_preserve(pic);
+  size_t ai = pic_gc_arena_preserve(pic);
   pic_value val;
 
   val = pic_cons(pic, obj1, pic_list1(pic, obj2));
@@ -97,7 +97,7 @@ pic_list2(pic_state *pic, pic_value obj1, pic_value obj2)
 pic_value
 pic_list3(pic_state *pic, pic_value obj1, pic_value obj2, pic_value obj3)
 {
-  int ai = pic_gc_arena_preserve(pic);
+  size_t ai = pic_gc_arena_preserve(pic);
   pic_value val;
 
   val = pic_cons(pic, obj1, pic_list2(pic, obj2, obj3));
@@ -110,7 +110,7 @@ pic_list3(pic_state *pic, pic_value obj1, pic_value obj2, pic_value obj3)
 pic_value
 pic_list4(pic_state *pic, pic_value obj1, pic_value obj2, pic_value obj3, pic_value obj4)
 {
-  int ai = pic_gc_arena_preserve(pic);
+  size_t ai = pic_gc_arena_preserve(pic);
   pic_value val;
 
   val = pic_cons(pic, obj1, pic_list3(pic, obj2, obj3, obj4));
@@ -123,7 +123,7 @@ pic_list4(pic_state *pic, pic_value obj1, pic_value obj2, pic_value obj3, pic_va
 pic_value
 pic_list5(pic_state *pic, pic_value obj1, pic_value obj2, pic_value obj3, pic_value obj4, pic_value obj5)
 {
-  int ai = pic_gc_arena_preserve(pic);
+  size_t ai = pic_gc_arena_preserve(pic);
   pic_value val;
 
   val = pic_cons(pic, obj1, pic_list4(pic, obj2, obj3, obj4, obj5));
@@ -136,7 +136,7 @@ pic_list5(pic_state *pic, pic_value obj1, pic_value obj2, pic_value obj3, pic_va
 pic_value
 pic_list6(pic_state *pic, pic_value obj1, pic_value obj2, pic_value obj3, pic_value obj4, pic_value obj5, pic_value obj6)
 {
-  int ai = pic_gc_arena_preserve(pic);
+  size_t ai = pic_gc_arena_preserve(pic);
   pic_value val;
 
   val = pic_cons(pic, obj1, pic_list5(pic, obj2, obj3, obj4, obj5, obj6));
@@ -149,7 +149,7 @@ pic_list6(pic_state *pic, pic_value obj1, pic_value obj2, pic_value obj3, pic_va
 pic_value
 pic_list7(pic_state *pic, pic_value obj1, pic_value obj2, pic_value obj3, pic_value obj4, pic_value obj5, pic_value obj6, pic_value obj7)
 {
-  int ai = pic_gc_arena_preserve(pic);
+  size_t ai = pic_gc_arena_preserve(pic);
   pic_value val;
 
   val = pic_cons(pic, obj1, pic_list6(pic, obj2, obj3, obj4, obj5, obj6, obj7));
@@ -205,7 +205,7 @@ pic_length(pic_state *pic, pic_value obj)
 pic_value
 pic_reverse(pic_state *pic, pic_value list)
 {
-  int ai = pic_gc_arena_preserve(pic);
+  size_t ai = pic_gc_arena_preserve(pic);
   pic_value v, acc;
 
   acc = pic_nil_value();
@@ -221,7 +221,7 @@ pic_reverse(pic_state *pic, pic_value list)
 pic_value
 pic_append(pic_state *pic, pic_value xs, pic_value ys)
 {
-  int ai = pic_gc_arena_preserve(pic);
+  size_t ai = pic_gc_arena_preserve(pic);
   pic_value x;
 
   xs = pic_reverse(pic, xs);
diff --git a/src/read.c b/src/read.c
index e768c10e..030ee0e3 100644
--- a/src/read.c
+++ b/src/read.c
@@ -217,7 +217,7 @@ read_datum(int tok, yyscan_t scanner)
 static pic_value
 read(int tok, yyscan_t scanner)
 {
-  int ai = pic_gc_arena_preserve(pic);
+  size_t ai = pic_gc_arena_preserve(pic);
   pic_value val;
 
   val = read_datum(tok, scanner);
diff --git a/src/state.c b/src/state.c
index 37fab568..a9a13ba8 100644
--- a/src/state.c
+++ b/src/state.c
@@ -18,7 +18,7 @@ pic_open(int argc, char *argv[], char **envp)
   char t;
 
   pic_state *pic;
-  int ai;
+  size_t ai;
 
   pic = (pic_state *)malloc(sizeof(pic_state));
 
@@ -70,6 +70,8 @@ pic_open(int argc, char *argv[], char **envp)
   pic->try_jmps = NULL;
 
   /* GC arena */
+  pic->arena = (struct pic_object **)calloc(PIC_ARENA_SIZE, sizeof(struct pic_object **));
+  pic->arena_size = PIC_ARENA_SIZE;
   pic->arena_idx = 0;
 
   /* native stack marker */
@@ -153,6 +155,9 @@ pic_close(pic_state *pic)
   xh_destroy(&pic->global_tbl);
   xh_destroy(&pic->macros);
 
+  /* free GC arena */
+  free(pic->arena);
+
   /* free symbol names */
   xh_begin(&it, &pic->sym_names);
   while (xh_next(&it)) {
diff --git a/src/system.c b/src/system.c
index b5108af1..efd53f48 100644
--- a/src/system.c
+++ b/src/system.c
@@ -18,7 +18,7 @@ pic_system_cmdline(pic_state *pic)
   pic_get_args(pic, "");
 
   for (i = 0; i < pic->argc; ++i) {
-    int ai = pic_gc_arena_preserve(pic);
+    size_t ai = pic_gc_arena_preserve(pic);
 
     v = pic_cons(pic, pic_obj_value(pic_str_new_cstr(pic, pic->argv[i])), v);
     pic_gc_arena_restore(pic, ai);
@@ -95,7 +95,7 @@ pic_system_getenvs(pic_state *pic)
 {
   char **envp;
   pic_value data = pic_nil_value();
-  int ai = pic_gc_arena_preserve(pic);
+  size_t ai = pic_gc_arena_preserve(pic);
 
   pic_get_args(pic, "");
 
diff --git a/src/vm.c b/src/vm.c
index 760a63eb..6c89d8a8 100644
--- a/src/vm.c
+++ b/src/vm.c
@@ -514,7 +514,7 @@ pic_value
 pic_apply(pic_state *pic, struct pic_proc *proc, pic_value argv)
 {
   pic_code c;
-  int ai = pic_gc_arena_preserve(pic);
+  size_t ai = pic_gc_arena_preserve(pic);
   size_t argc, i;
   pic_code boot[2];