diff --git a/include/mruby/internal.h b/include/mruby/internal.h
index 02500521b..5215d0e51 100644
--- a/include/mruby/internal.h
+++ b/include/mruby/internal.h
@@ -182,6 +182,7 @@ mrb_float mrb_bint_as_float(mrb_state *mrb, mrb_value x);
 mrb_int mrb_bint_as_int(mrb_state *mrb, mrb_value x);
 mrb_int mrb_bint_cmp(mrb_state *mrb, mrb_value x, mrb_value y);
 void mrb_gc_free_bint(mrb_state *mrb, struct RBasic *x);
+void mrb_bint_copy(mrb_state *mrb, mrb_value x, mrb_value y);
 #endif
 
 #endif  /* MRUBY_INTERNAL_H */
diff --git a/mrbgems/mruby-bigint/core/bigint.c b/mrbgems/mruby-bigint/core/bigint.c
index 4aba1c340..53464a3ab 100644
--- a/mrbgems/mruby-bigint/core/bigint.c
+++ b/mrbgems/mruby-bigint/core/bigint.c
@@ -23,7 +23,6 @@ mpz_init(mrb_state *mrb, mpz_t *s)
   s->sz=2;
 }
 
-#if 0
 static void
 mpz_init_set(mrb_state *mrb, mpz_t *s, mpz_t *t)
 {
@@ -34,7 +33,6 @@ mpz_init_set(mrb_state *mrb, mpz_t *s, mpz_t *t)
   s->sn = t->sn;
   s->sz = t->sz;
 }
-#endif
 
 static void
 mpz_set_int(mrb_state *mrb, mpz_t *y, mrb_int v)
@@ -1450,3 +1448,11 @@ mrb_bint_rshift(mrb_state *mrb, mrb_value x, mrb_int width)
   }
   return bint_norm(mrb, b2);
 }
+
+void
+mrb_bint_copy(mrb_state *mrb, mrb_value x, mrb_value y)
+{
+  struct RBigint *b = RBIGINT(x);
+  struct RBigint *b2 = RBIGINT(y);
+  mpz_init_set(mrb, &b->mp, &b2->mp);
+}
diff --git a/src/class.c b/src/class.c
index 483a88ee0..b8a2268c2 100644
--- a/src/class.c
+++ b/src/class.c
@@ -2737,6 +2737,11 @@ init_copy(mrb_state *mrb, mrb_value dest, mrb_value obj)
     case MRB_TT_ISTRUCT:
       mrb_istruct_copy(dest, obj);
       break;
+#ifdef MRB_USE_BIGINT
+    case MRB_TT_BIGINT:
+      mrb_bint_copy(mrb, dest, obj);
+      break;
+#endif
 
     default:
       break;