diff --git a/mrbgems/mruby-io/mrblib/io.rb b/mrbgems/mruby-io/mrblib/io.rb
index 20ae8d993..5f62784b9 100644
--- a/mrbgems/mruby-io/mrblib/io.rb
+++ b/mrbgems/mruby-io/mrblib/io.rb
@@ -5,10 +5,6 @@ class IOError < StandardError; end
 class EOFError < IOError; end
 
 class IO
-  SEEK_SET = 0
-  SEEK_CUR = 1
-  SEEK_END = 2
-
   def self.open(*args, &block)
     io = self.new(*args)
 
diff --git a/mrbgems/mruby-io/src/io.c b/mrbgems/mruby-io/src/io.c
index dd30668b4..0eb23fe0f 100644
--- a/mrbgems/mruby-io/src/io.c
+++ b/mrbgems/mruby-io/src/io.c
@@ -1578,6 +1578,10 @@ mrb_init_io(mrb_state *mrb)
   mrb_define_method(mrb, io, "pread",      mrb_io_pread,      MRB_ARGS_ANY());    /* ruby 2.5 feature */
   mrb_define_method(mrb, io, "pwrite",     mrb_io_pwrite,     MRB_ARGS_ANY());    /* ruby 2.5 feature */
 
+  mrb_define_const_id(mrb, io, MRB_SYM(SEEK_SET), mrb_fixnum_value(SEEK_SET));
+  mrb_define_const_id(mrb, io, MRB_SYM(SEEK_CUR), mrb_fixnum_value(SEEK_CUR));
+  mrb_define_const_id(mrb, io, MRB_SYM(SEEK_END), mrb_fixnum_value(SEEK_END));
+
   mrb_define_method(mrb, io, "_read_buf",  mrb_io_read_buf,   MRB_ARGS_NONE());
   mrb_define_method(mrb, io, "_readchar",  mrb_io_readchar,   MRB_ARGS_NONE());
   mrb_define_class_method(mrb, io, "_bufread",   mrb_io_bufread,    MRB_ARGS_REQ(2));