diff --git a/gc/mmtk/mmtk.c b/gc/mmtk/mmtk.c
index 6547a6c..113e378 100644
--- a/gc/mmtk/mmtk.c
+++ b/gc/mmtk/mmtk.c
@@ -27,7 +27,6 @@
 
 #if RACTOR_CHECK_MODE
 # define RVALUE_SUFFIX_SIZE sizeof(VALUE)
-void rb_ractor_setup_belonging(VALUE obj);
 #else
 # define RVALUE_SUFFIX_SIZE 0
 #endif
@@ -930,10 +929,6 @@ rb_gc_impl_new_obj(void *objspace_ptr, void *cache_ptr, VALUE klass, VALUE flags
 
     objspace->total_allocated_objects++;
 
-#if RACTOR_CHECK_MODE
-    rb_ractor_setup_belonging((VALUE)alloc_obj);
-#endif
-
     return (VALUE)alloc_obj;
 }