diff --git a/include/picrin.h b/include/picrin.h
index 7258fb7a..dbe58f12 100644
--- a/include/picrin.h
+++ b/include/picrin.h
@@ -151,7 +151,7 @@ struct pic_string *pic_str_new_cstr(pic_state *, const char *);
 
 struct pic_vector *pic_vec_new(pic_state *, size_t);
 struct pic_vector *pic_vec_new_from_list(pic_state *, pic_value);
-void pic_vec_extend_ip(pic_state *, struct pic_vector *, int);
+void pic_vec_extend_ip(pic_state *, struct pic_vector *, size_t);
 
 int pic_parse_file(pic_state *, FILE *file, pic_value *);
 int pic_parse_cstr(pic_state *, const char *, pic_value *);
diff --git a/include/picrin/blob.h b/include/picrin/blob.h
index 7b99885d..6b52d9e3 100644
--- a/include/picrin/blob.h
+++ b/include/picrin/blob.h
@@ -12,13 +12,13 @@ extern "C" {
 struct pic_blob {
   PIC_OBJECT_HEADER
   char *data;
-  int len;
+  size_t len;
 };
 
 #define pic_blob_p(v) (pic_type(v) == PIC_TT_BLOB)
 #define pic_blob_ptr(v) ((struct pic_blob *)pic_ptr(v))
 
-struct pic_blob *pic_blob_new(pic_state *, char *, int len);
+struct pic_blob *pic_blob_new(pic_state *, char *, size_t len);
 
 #if defined(__cplusplus)
 }
diff --git a/include/picrin/parse.h b/include/picrin/parse.h
index a1784f87..44405639 100644
--- a/include/picrin/parse.h
+++ b/include/picrin/parse.h
@@ -16,7 +16,7 @@ struct parser_control {
   bool incomp;
   int yynerrs;
   struct pic_vector *yy_arena;
-  int yy_arena_idx;
+  size_t yy_arena_idx;
 };
 
 #if defined(__cplusplus)
diff --git a/src/blob.c b/src/blob.c
index 50304a19..afa50747 100644
--- a/src/blob.c
+++ b/src/blob.c
@@ -8,7 +8,7 @@
 #include "picrin/blob.h"
 
 struct pic_blob *
-pic_blob_new(pic_state *pic, char *dat, int len)
+pic_blob_new(pic_state *pic, char *dat, size_t len)
 {
   struct pic_blob *bv;
 
diff --git a/src/codegen.c b/src/codegen.c
index d580c9e2..356fba0d 100644
--- a/src/codegen.c
+++ b/src/codegen.c
@@ -36,10 +36,10 @@ new_irep(pic_state *pic)
 }
 
 static pic_sym *
-analyze_args(pic_state *pic, pic_value args, bool *varg, size_t *argc, size_t *localc)
+analyze_args(pic_state *pic, pic_value args, bool *varg, int *argc, int *localc)
 {
   pic_sym *syms = (pic_sym *)pic_alloc(pic, sizeof(pic_sym));
-  size_t i = 1, l = 0;
+  int i = 1, l = 0;
   pic_value v;
 
   *varg = false;
@@ -78,7 +78,7 @@ static bool
 valid_formal(pic_state *pic, pic_value formal)
 {
   bool varg;
-  size_t argc, localc;
+  int argc, localc;
   pic_sym *syms;
 
   syms = analyze_args(pic, formal, &varg, &argc, &localc);
@@ -94,7 +94,7 @@ valid_formal(pic_state *pic, pic_value formal)
 typedef struct analyze_scope {
   /* rest args variable is counted by localc */
   bool varg;
-  size_t argc, localc;
+  int argc, localc;
   /* if variable v is captured, then xh_get(var_tbl, v) == 1 */
   struct xhash *var_tbl;
   pic_sym *vars;
@@ -665,7 +665,7 @@ pic_analyze(pic_state *pic, pic_value obj)
 typedef struct resolver_scope {
   int depth;
   bool varg;
-  size_t argc, localc;
+  int argc, localc;
   struct xhash *cvs, *lvs;
   unsigned cv_num;
 
@@ -769,7 +769,7 @@ resolve_gref(resolver_state *state, pic_sym sym)
   pic_state *pic = state->pic;
   const char *name = pic_symbol_name(pic, sym);
   struct xh_entry *e;
-  int i;
+  size_t i;
 
   if ((e = xh_get(pic->global_tbl, name))) {
     i = e->val;
@@ -911,7 +911,7 @@ pic_resolve(pic_state *pic, pic_value obj)
 typedef struct codegen_context {
   bool varg;
   /* rest args variable is counted by localc */
-  size_t argc, localc;
+  int argc, localc;
   /* closed variable table */
   unsigned *cv_tbl, cv_num;
   /* actual bit code sequence */
diff --git a/src/gc.c b/src/gc.c
index 35b4e45f..a71af550 100644
--- a/src/gc.c
+++ b/src/gc.c
@@ -342,7 +342,7 @@ gc_mark_object(pic_state *pic, struct pic_object *obj)
     break;
   }
   case PIC_TT_VECTOR: {
-    int i;
+    size_t i;
     for (i = 0; i < ((struct pic_vector *)obj)->len; ++i) {
       gc_mark(pic, ((struct pic_vector *)obj)->data[i]);
     }
@@ -355,7 +355,8 @@ gc_mark_object(pic_state *pic, struct pic_object *obj)
     struct pic_cont *cont = (struct pic_cont *)obj;
     pic_value *stack;
     pic_callinfo *ci;
-    int i;
+    size_t i;
+    int j;
 
     /* block */
     gc_mark_block(pic, cont->blk);
@@ -378,8 +379,8 @@ gc_mark_object(pic_state *pic, struct pic_object *obj)
     }
 
     /* arena */
-    for (i = 0; i < cont->arena_idx; ++i) {
-      gc_mark_object(pic, cont->arena[i]);
+    for (j = 0; j < cont->arena_idx; ++j) {
+      gc_mark_object(pic, cont->arena[j]);
     }
 
     gc_mark(pic, cont->result);
@@ -403,7 +404,7 @@ gc_mark_object(pic_state *pic, struct pic_object *obj)
       gc_mark_object(pic, (struct pic_object *)senv->up);
     }
     if (senv->stx) {
-      int i;
+      size_t i;
 
       for (i = 0; i < senv->xlen; ++i) {
 	gc_mark_object(pic, (struct pic_object *)senv->stx[i]);
@@ -433,7 +434,7 @@ gc_mark_object(pic_state *pic, struct pic_object *obj)
   }
   case PIC_TT_IREP: {
     struct pic_irep *irep = (struct pic_irep *)obj;
-    int i;
+    size_t i;
 
     for (i = 0; i < irep->ilen; ++i) {
       gc_mark_object(pic, (struct pic_object *)irep->irep[i]);
@@ -472,7 +473,8 @@ gc_mark_phase(pic_state *pic)
 {
   pic_value *stack;
   pic_callinfo *ci;
-  int i;
+  size_t i;
+  int j;
 
   /* block */
   gc_mark_block(pic, pic->blk);
@@ -495,8 +497,8 @@ gc_mark_phase(pic_state *pic)
   }
 
   /* arena */
-  for (i = 0; i < pic->arena_idx; ++i) {
-    gc_mark_object(pic, pic->arena[i]);
+  for (j = 0; j < pic->arena_idx; ++j) {
+    gc_mark_object(pic, pic->arena[j]);
   }
 
   /* global variables */
diff --git a/src/macro.c b/src/macro.c
index 3ad0a139..af011343 100644
--- a/src/macro.c
+++ b/src/macro.c
@@ -200,7 +200,7 @@ pic_import(pic_state *pic, pic_value spec)
       xh_put(pic->lib->senv->tbl, it.e->key, it.e->val);
     }
     else {                /* syntax object */
-      int idx;
+      size_t idx;
       struct pic_senv *senv = pic->lib->senv;
 
       idx = senv->xlen;
@@ -233,7 +233,7 @@ pic_defsyntax(pic_state *pic, const char *name, struct pic_proc *macro, struct p
 {
   struct pic_syntax *stx;
   struct pic_senv *global_senv = pic->lib->senv;
-  int idx;
+  size_t idx;
 
   stx = pic_syntax_new_macro(pic, pic_intern_cstr(pic, name), macro, mac_env);
 
diff --git a/src/number.c b/src/number.c
index 321bab31..e5cdde1c 100644
--- a/src/number.c
+++ b/src/number.c
@@ -123,7 +123,7 @@ pic_number_nan_p(pic_state *pic)
   {							\
     size_t argc;					\
     pic_value *argv;					\
-    int i;						\
+    size_t i;						\
     double f,g;						\
     							\
     pic_get_args(pic, "ff*", &f, &g, &argc, &argv);	\
@@ -207,7 +207,7 @@ pic_number_max(pic_state *pic)
 {
   size_t argc;
   pic_value *argv;
-  int i;
+  size_t i;
   double f;
   bool e = true;
 
@@ -235,7 +235,7 @@ pic_number_min(pic_state *pic)
 {
   size_t argc;
   pic_value *argv;
-  int i;
+  size_t i;
   double f;
   bool e = true;
 
@@ -264,7 +264,7 @@ pic_number_min(pic_state *pic)
   {                                                             \
     size_t argc;                                                \
     pic_value *argv;                                            \
-    int i;                                                      \
+    size_t i;                                                   \
     double f;                                                   \
     bool e = true;                                              \
                                                                 \
@@ -296,7 +296,7 @@ DEFINE_ARITH_OP(*, mul, 1)
   {                                                                     \
    size_t argc;                                                         \
    pic_value *argv;                                                     \
-   int i;                                                               \
+   size_t i;                                                            \
    double f;                                                            \
    bool e;                                                              \
                                                                         \
diff --git a/src/pair.c b/src/pair.c
index 30291d46..4495b641 100644
--- a/src/pair.c
+++ b/src/pair.c
@@ -81,7 +81,7 @@ pic_value
 pic_list_from_array(pic_state *pic, size_t c, pic_value *vs)
 {
   pic_value v;
-  int i;
+  size_t i;
 
   v = pic_nil_value();
   for (i = 0; i < c; ++i) {
diff --git a/src/state.c b/src/state.c
index bbe6c253..8236d2e9 100644
--- a/src/state.c
+++ b/src/state.c
@@ -126,7 +126,7 @@ pic_open(int argc, char *argv[], char **envp)
 void
 pic_close(pic_state *pic)
 {
-  int i;
+  size_t i;
 
   /* free global stacks */
   free(pic->stbase);
diff --git a/src/string.c b/src/string.c
index 1b0c84ac..a9cddcb1 100644
--- a/src/string.c
+++ b/src/string.c
@@ -95,7 +95,7 @@ pic_str_string_set(pic_state *pic)
   {							\
     size_t argc;					\
     pic_value *argv;					\
-    int i;						\
+    size_t i;						\
 							\
     pic_get_args(pic, "*", &argc, &argv);		\
 							\
diff --git a/src/vector.c b/src/vector.c
index d8b34f3c..dcfe1bed 100644
--- a/src/vector.c
+++ b/src/vector.c
@@ -9,7 +9,7 @@ struct pic_vector *
 pic_vec_new(pic_state *pic, size_t len)
 {
   struct pic_vector *vec;
-  int i;
+  size_t i;
 
   vec = (struct pic_vector *)pic_obj_alloc(pic, sizeof(struct pic_vector), PIC_TT_VECTOR);
   vec->len = len;
@@ -24,7 +24,7 @@ struct pic_vector *
 pic_vec_new_from_list(pic_state *pic, pic_value data)
 {
   struct pic_vector *vec;
-  int i, len;
+  size_t i, len;
 
   len = pic_length(pic, data);
 
@@ -37,9 +37,9 @@ pic_vec_new_from_list(pic_state *pic, pic_value data)
 }
 
 void
-pic_vec_extend_ip(pic_state *pic, struct pic_vector *vec, int size)
+pic_vec_extend_ip(pic_state *pic, struct pic_vector *vec, size_t size)
 {
-  int len, i;
+  size_t len, i;
 
   len = vec->len;
   vec->len = size;
@@ -63,14 +63,15 @@ static pic_value
 pic_vec_make_vector(pic_state *pic)
 {
   pic_value v;
-  int k, n, i;
+  int n, k;
+  size_t i;
   struct pic_vector *vec;
 
   n = pic_get_args(pic, "i|o", &k, &v);
 
   vec = pic_vec_new(pic, k);
   if (n == 3) {
-    for (i = 0; i < k; ++i) {
+    for (i = 0; i < (size_t)k; ++i) {
       vec->data[i] = v;
     }
   }
@@ -95,7 +96,7 @@ pic_vec_vector_ref(pic_state *pic)
 
   pic_get_args(pic, "vi", &v, &k);
 
-  if (k < 0 || v->len <= k) {
+  if (k < 0 || v->len <= (size_t)k) {
     pic_error(pic, "vector-ref: index out of range");
   }
   return v->data[k];
@@ -110,7 +111,7 @@ pic_vec_vector_set(pic_state *pic)
 
   pic_get_args(pic, "vio", &v, &k, &o);
 
-  if (k < 0 || v->len <= k) {
+  if (k < 0 || v->len <= (size_t)k) {
     pic_error(pic, "vector-set!: index out of range");
   }
   v->data[k] = o;
diff --git a/src/vm.c b/src/vm.c
index 64fa4aaf..b1160196 100644
--- a/src/vm.c
+++ b/src/vm.c
@@ -630,7 +630,7 @@ pic_apply(pic_state *pic, struct pic_proc *proc, pic_value argv)
       }
     }
     CASE(OP_TAILCALL) {
-      int argc;
+      int i, argc;
       pic_value *argv;
 
       argc = c.u.i;
diff --git a/src/write.c b/src/write.c
index 026cb391..614fcb9e 100644
--- a/src/write.c
+++ b/src/write.c
@@ -30,7 +30,7 @@ write_pair(pic_state *pic, struct pic_pair *pair)
 static void
 write_str(pic_state *pic, struct pic_string *str)
 {
-  int i;
+  size_t i;
   const char *cstr = str->str;
 
   for (i = 0; i < str->len; ++i) {
@@ -44,7 +44,7 @@ write_str(pic_state *pic, struct pic_string *str)
 static void
 write(pic_state *pic, pic_value obj)
 {
-  int i;
+  size_t i;
 
   switch (pic_type(obj)) {
   case PIC_TT_NIL: