diff --git a/mrbgems/mruby-sleep/src/sleep.c b/mrbgems/mruby-sleep/src/sleep.c
index 10b9958ce..8f434957d 100644
--- a/mrbgems/mruby-sleep/src/sleep.c
+++ b/mrbgems/mruby-sleep/src/sleep.c
@@ -129,8 +129,8 @@ f_usleep(mrb_state *mrb, mrb_value self)
 void
 mrb_mruby_sleep_gem_init(mrb_state *mrb)
 {
-  mrb_define_method_id(mrb, mrb->kernel_module, MRB_SYM(sleep),   f_sleep,   MRB_ARGS_REQ(1));
-  mrb_define_method_id(mrb, mrb->kernel_module, MRB_SYM(usleep),  f_usleep,  MRB_ARGS_REQ(1));
+  mrb_define_private_method_id(mrb, mrb->kernel_module, MRB_SYM(sleep),   f_sleep,   MRB_ARGS_REQ(1));
+  mrb_define_private_method_id(mrb, mrb->kernel_module, MRB_SYM(usleep),  f_usleep,  MRB_ARGS_REQ(1));
 }
 
 void