diff --git a/include/mruby/internal.h b/include/mruby/internal.h
index 1f991c414..68af07f3c 100644
--- a/include/mruby/internal.h
+++ b/include/mruby/internal.h
@@ -185,6 +185,7 @@ 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);
+size_t mrb_bint_memsize(mrb_value x);
 #endif
 
 #endif  /* MRUBY_INTERNAL_H */
diff --git a/mrbgems/mruby-bigint/core/bigint.c b/mrbgems/mruby-bigint/core/bigint.c
index 40d59eb38..ce91232f2 100644
--- a/mrbgems/mruby-bigint/core/bigint.c
+++ b/mrbgems/mruby-bigint/core/bigint.c
@@ -1492,3 +1492,10 @@ mrb_bint_copy(mrb_state *mrb, mrb_value x, mrb_value y)
   struct RBigint *b2 = RBIGINT(y);
   mpz_init_set(mrb, &b->mp, &b2->mp);
 }
+
+size_t
+mrb_bint_memsize(mrb_value x)
+{
+  struct RBigint *b = RBIGINT(x);
+  return b->mp.sz * sizeof(mp_limb);
+}
diff --git a/mrbgems/mruby-os-memsize/src/memsize.c b/mrbgems/mruby-os-memsize/src/memsize.c
index 121cbb060..9206a8ac5 100644
--- a/mrbgems/mruby-os-memsize/src/memsize.c
+++ b/mrbgems/mruby-os-memsize/src/memsize.c
@@ -146,6 +146,9 @@ os_memsize_of_object(mrb_state* mrb, mrb_value obj)
 #endif
       break;
     case MRB_TT_BIGINT:
+#if defined(MRB_USE_BIGINT)
+      size += mrb_bint_memsize(obj);
+#endif
     case MRB_TT_DATA:
     case MRB_TT_ISTRUCT:
       size += mrb_objspace_page_slot_size();