diff --git a/debug_counter.h b/debug_counter.h
index b29d5ef560..3fcb562f65 100644
--- a/debug_counter.h
+++ b/debug_counter.h
@@ -119,6 +119,7 @@ RB_DEBUG_COUNTER(gc_major_nofree)
 RB_DEBUG_COUNTER(gc_major_oldgen)
 RB_DEBUG_COUNTER(gc_major_shady)
 RB_DEBUG_COUNTER(gc_major_force)
+RB_DEBUG_COUNTER(gc_major_oldmalloc)
 
 /* object allocation counts:
  *
diff --git a/gc.c b/gc.c
index c4d12770b2..c3c4f4ea57 100644
--- a/gc.c
+++ b/gc.c
@@ -6538,11 +6538,15 @@ gc_start(rb_objspace_t *objspace, int reason)
 
 #if USE_DEBUG_COUNTER
     RB_DEBUG_COUNTER_INC(gc_count);
+
     if (reason & GPR_FLAG_MAJOR_MASK) {
         (void)RB_DEBUG_COUNTER_INC_IF(gc_major_nofree, reason & GPR_FLAG_MAJOR_BY_NOFREE);
         (void)RB_DEBUG_COUNTER_INC_IF(gc_major_oldgen, reason & GPR_FLAG_MAJOR_BY_OLDGEN);
         (void)RB_DEBUG_COUNTER_INC_IF(gc_major_shady,  reason & GPR_FLAG_MAJOR_BY_SHADY);
         (void)RB_DEBUG_COUNTER_INC_IF(gc_major_force,  reason & GPR_FLAG_MAJOR_BY_FORCE);
+#if RGENGC_ESTIMATE_OLDMALLOC
+        (void)RB_DEBUG_COUNTER_INC_IF(gc_major_oldmalloc, reason & GPR_FLAG_MAJOR_BY_OLDMALLOC);
+#endif
     }
     else {
         (void)RB_DEBUG_COUNTER_INC_IF(gc_minor_newobj, reason & GPR_FLAG_NEWOBJ);