diff --git a/src/bool.c b/src/bool.c
index 904a21d6..8ed9cc02 100644
--- a/src/bool.c
+++ b/src/bool.c
@@ -10,33 +10,87 @@
 #include "picrin/blob.h"
 #include "picrin/string.h"
 
-bool
-pic_equal_p(pic_state *pic, pic_value x, pic_value y)
+bool pic_string_equal_p(struct pic_string *str1, struct pic_string *str2)
 {
+  return pic_strcmp(str1, str2) == 0;
+}
+
+bool pic_blob_equal_p(struct pic_blob *blob1, struct pic_blob *blob2)
+{
+  if(blob1->len != blob2->len){
+    return false;
+  }
+  size_t i;
+  for(i = 0; i < blob1->len; ++i){
+    if(blob1->data[i] != blob2->data[i])
+      return false;
+  }
+  return true;  
+}
+
+bool
+pic_internal_equal_p(pic_state *pic, pic_value x, pic_value y, size_t depth, xhash *ht)
+{
+
+  if (depth > 10){
+    if(depth > 200){
+      pic_errorf(pic, "Stack overflow in equal\n");
+    }
+    if (NULL == ht){
+      xh_init_ptr(ht, sizeof(void *));
+    }
+    switch(pic_type(x)){
+    case PIC_TT_PAIR:
+    case PIC_TT_VECTOR:{
+      xh_entry *e = xh_get(ht, pic_obj_ptr(x));
+      if(e){
+        /* `x' was seen already.  */
+        return true;
+      }else{
+        xh_put(ht, pic_obj_ptr(x), NULL);
+      }
+    }
+    default:;
+    }
+  }
+
   enum pic_tt type;
+  pic_value local = pic_nil_value();
+  size_t rapid_count = 0;
+
+ LOOP:
 
   if (pic_eqv_p(x, y))
     return true;
-
+  
   type = pic_type(x);
-  if (type != pic_type(y))
-    return false;
+  
+  if (type != pic_type(y)){
+    return false;    
+  }
+  
   switch (type) {
   case PIC_TT_PAIR:
-    return pic_equal_p(pic, pic_car(pic, x), pic_car(pic, y))
-      && pic_equal_p(pic, pic_cdr(pic, x), pic_cdr(pic, y));
-  case PIC_TT_BLOB: {
-    int i;
-    struct pic_blob *v1 = pic_blob_ptr(x), *v2 = pic_blob_ptr(y);
-    if(v1->len != v2->len){
+    if(pic_nil_p(local)){
+      local = x;
+    }
+    if(pic_internal_equal_p(pic, pic_car(pic, x), pic_car(pic, y), depth + 1, ht)){
+      x = pic_cdr(pic, x);
+      y = pic_cdr(pic, y);
+      ++rapid_count;
+      
+      if(rapid_count == 2){
+        rapid_count = 0;
+        local = pic_cdr(pic, local);
+        if (pic_eq_p(local, x)) {
+          return true;
+        }
+      }
+      goto LOOP;
+    }else{
       return false;
     }
-    for(i = 0; i < v1->len; ++i){
-      if(v1->data[i] != v2->data[i])
-        return false;
-    }
-    return true;
-  }
+      
   case PIC_TT_VECTOR:{
     size_t i;
     struct pic_vector *v1 = pic_vec_ptr(x), *v2 = pic_vec_ptr(y);
@@ -45,18 +99,28 @@ pic_equal_p(pic_state *pic, pic_value x, pic_value y)
       return false;
     }
     for(i = 0; i < v1->len; ++i){
-      if(! pic_equal_p(pic, v1->data[i], v2->data[i]))
-        return false;
+      if(! pic_internal_equal_p(pic, v1->data[i], v2->data[i], depth + 1, ht)){
+        return false;          
+        }
     }
     return true;
   }
+  case PIC_TT_BLOB: 
+    return pic_blob_equal_p(pic_blob_ptr(x), pic_blob_ptr(y));
   case PIC_TT_STRING:
-    return pic_strcmp(pic_str_ptr(x), pic_str_ptr(y)) == 0;
+    return pic_string_equal_p(pic_str_ptr(x), pic_str_ptr(y));
   default:
     return false;
   }
 }
 
+bool
+pic_equal_p(pic_state *pic, pic_value x, pic_value y){
+  xhash ht;
+  xh_init_ptr(&ht, 0);
+  return pic_internal_equal_p(pic, x, y, 0, &ht);
+}
+
 static pic_value
 pic_bool_eq_p(pic_state *pic)
 {