diff --git a/include/picrin.h b/include/picrin.h
index 8c7cb34f..d5589726 100644
--- a/include/picrin.h
+++ b/include/picrin.h
@@ -22,6 +22,7 @@ struct pic_block {
   struct pic_block *prev;
   int depth;
   struct pic_proc *in, *out;
+  unsigned refcnt;
 };
 
 typedef struct {
diff --git a/include/picrin/cont.h b/include/picrin/cont.h
index f1b9cb80..7864e320 100644
--- a/include/picrin/cont.h
+++ b/include/picrin/cont.h
@@ -22,4 +22,21 @@ struct pic_cont {
   pic_value result;
 };
 
+#define PIC_BLK_INCREF(pic,blk) do {		\
+    (blk)->refcnt++;				\
+  } while (0)
+
+#define PIC_BLK_DECREF(pic,blk) do {			\
+    struct pic_block *_a = (blk), *_b;			\
+    while (_a) {					\
+      if (! --_a->refcnt) {				\
+	_b = _a->prev;					\
+	pic_free((pic), _a);				\
+	_a = _b;					\
+      } else {						\
+	break;						\
+      }							\
+    }							\
+  } while (0)
+
 #endif
diff --git a/src/cont.c b/src/cont.c
index 0f9229ef..387152c7 100644
--- a/src/cont.c
+++ b/src/cont.c
@@ -31,6 +31,7 @@ save_cont(pic_state *pic)
   cont = (struct pic_cont *)pic_obj_alloc(pic, sizeof(struct pic_cont), PIC_TT_CONT);
 
   cont->blk = pic->blk;
+  PIC_BLK_INCREF(pic, cont->blk);
 
   cont->stk_len = native_stack_length(pic, &pos);
   cont->stk_pos = pos;
@@ -77,6 +78,8 @@ restore_cont(pic_state *pic, struct pic_cont *cont)
     if (&v > cont->stk_pos + cont->stk_len) native_stack_extend(pic, cont);
   }
 
+  PIC_BLK_DECREF(pic, pic->blk);
+  PIC_BLK_INCREF(pic, cont->blk);
   pic->blk = cont->blk;
 
   pic->sp = cont->sp;
@@ -122,7 +125,7 @@ cont_call(pic_state *pic)
   cont = (struct pic_cont *)pic_ptr(proc->env->values[0]);
   cont->result = v;
 
-  /* execute winded handlers */
+  /* execute guard handlers */
   walk_to_block(pic, pic->blk, cont->blk);
 
   restore_cont(pic, cont);
@@ -196,9 +199,12 @@ pic_cont_dynamic_wind(pic_state *pic)
     pic->blk->depth = here->depth + 1;
     pic->blk->in = in;
     pic->blk->out = out;
+    pic->blk->refcnt = 1;
+    PIC_BLK_INCREF(pic, here);
 
     v = pic_apply_argv(pic, thunk, 0);
 
+    PIC_BLK_DECREF(pic, pic->blk);
     pic->blk = here;
   }
   /* exit */
diff --git a/src/gc.c b/src/gc.c
index 152cf061..74bdb322 100644
--- a/src/gc.c
+++ b/src/gc.c
@@ -142,6 +142,19 @@ gc_alloc(pic_state *pic, size_t size)
 }
 
 static void gc_mark(pic_state *, pic_value);
+static void gc_mark_object(pic_state *pic, struct pic_object *obj);
+
+static void
+gc_mark_block(pic_state *pic, struct pic_block *blk)
+{
+  while (blk) {
+    if (blk->in)
+      gc_mark_object(pic, (struct pic_object *)blk->in);
+    if (blk->out)
+      gc_mark_object(pic, (struct pic_object *)blk->out);
+    blk = blk->prev;
+  }
+}
 
 static void
 gc_mark_object(pic_state *pic, struct pic_object *obj)
@@ -198,6 +211,9 @@ gc_mark_object(pic_state *pic, struct pic_object *obj)
     pic_callinfo *ci;
     int i;
 
+    /* block */
+    gc_mark_block(pic, cont->blk);
+
     /* stack */
     for (stack = cont->st_ptr; stack != cont->sp; ++stack) {
       gc_mark(pic, *stack);
@@ -249,6 +265,9 @@ gc_mark_phase(pic_state *pic)
   pic_callinfo *ci;
   int i;
 
+  /* block */
+  gc_mark_block(pic, pic->blk);
+
   /* stack */
   for (stack = pic->stbase; stack != pic->sp; ++stack) {
     gc_mark(pic, *stack);
@@ -336,6 +355,7 @@ 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_BLK_DECREF(pic, cont->blk);
     break;
   }
   case PIC_TT_NIL:
diff --git a/src/state.c b/src/state.c
index fbcf99b1..9e7d1d36 100644
--- a/src/state.c
+++ b/src/state.c
@@ -27,6 +27,7 @@ pic_open(int argc, char *argv[], char **envp)
   pic->blk->prev = NULL;
   pic->blk->depth = 0;
   pic->blk->in = pic->blk->out = NULL;
+  pic->blk->refcnt = 1;
 
   /* prepare VM stack */
   pic->stbase = pic->sp = (pic_value *)malloc(sizeof(pic_value) * PIC_STACK_SIZE);