diff --git a/mrbgems/mruby-io/mrblib/io.rb b/mrbgems/mruby-io/mrblib/io.rb
index 686e40d80..055794653 100644
--- a/mrbgems/mruby-io/mrblib/io.rb
+++ b/mrbgems/mruby-io/mrblib/io.rb
@@ -9,8 +9,6 @@ class IO
   SEEK_CUR = 1
   SEEK_END = 2
 
-  BUF_SIZE = 4096
-
   def self.open(*args, &block)
     io = self.new(*args)
 
@@ -144,11 +142,6 @@ class IO
     0
   end
 
-  def _read_buf
-    return @buf if @buf && @buf.bytesize > 0
-    sysread(BUF_SIZE, @buf)
-  end
-
   def ungetc(substr)
     raise TypeError.new "expect String, got #{substr.class}" unless substr.is_a?(String)
     if @buf.empty?
diff --git a/mrbgems/mruby-io/src/io.c b/mrbgems/mruby-io/src/io.c
index c7ee5045e..dd30668b4 100644
--- a/mrbgems/mruby-io/src/io.c
+++ b/mrbgems/mruby-io/src/io.c
@@ -1492,6 +1492,20 @@ mrb_io_bufread(mrb_state *mrb, mrb_value self)
   return io_bufread(mrb, str, len);
 }
 
+#define BUF_SIZE 4096
+
+static mrb_value
+mrb_io_read_buf(mrb_state *mrb, mrb_value self)
+{
+  mrb_value buf = mrb_iv_get(mrb, self, MRB_IVSYM(buf));
+  if (!mrb_nil_p(buf)) {
+    mrb_ensure_string_type(mrb, buf);
+    if (RSTRING_LEN(buf) > 0) return buf;
+  }
+  mrb_str_modify(mrb, RSTRING(buf));
+  return mrb_io_sysread_common(mrb, sysread, self, buf, BUF_SIZE, 0);
+}
+
 static mrb_value
 mrb_io_readchar(mrb_state *mrb, mrb_value self)
 {
@@ -1564,6 +1578,7 @@ 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_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));
 }
diff --git a/mrbgems/mruby-io/test/io.rb b/mrbgems/mruby-io/test/io.rb
index 88a89fee4..b4f67d701 100644
--- a/mrbgems/mruby-io/test/io.rb
+++ b/mrbgems/mruby-io/test/io.rb
@@ -140,9 +140,10 @@ assert('IO#read', '15.2.20.5.14') do
 end
 
 assert "IO#read(n) with n > IO::BUF_SIZE" do
+  buf_size = 4096  # copied from io.c
   skip "pipe is not supported on this platform" if MRubyIOTestUtil.win?
   IO.pipe do |r,w|
-    n = IO::BUF_SIZE+1
+    n = buf_size+1
     w.write 'a'*n
     assert_equal 'a'*n, r.read(n)
   end