diff --git a/mrbgems/mruby-string-ext/src/string.c b/mrbgems/mruby-string-ext/src/string.c
index e8ea2c3ab..6480e4a78 100644
--- a/mrbgems/mruby-string-ext/src/string.c
+++ b/mrbgems/mruby-string-ext/src/string.c
@@ -1359,6 +1359,29 @@ str_b(mrb_state *mrb, mrb_value self)
   return str;
 }
 
+static mrb_value
+str_force_encoding(mrb_state *mrb, mrb_value self)
+{
+  mrb_value enc;
+
+  mrb_get_args(mrb, "S", &enc);
+
+  struct RString *s = mrb_str_ptr(self);
+  if (ENC_COMP_P(enc, ENC_ASCII_8BIT) ||
+      ENC_COMP_P(enc, ENC_BINARY)) {
+    s->flags |= MRB_STR_BINARY;
+  }
+#ifdef MRB_UTF8_STRING
+  else if (ENC_COMP_P(enc, ENC_UTF8)) {
+    s->flags &= ~MRB_STR_BINARY;
+  }
+#endif
+  else {
+    mrb_raisef(mrb, E_ARGUMENT_ERROR, "unknown encoding name - %v", enc);
+  }
+  return self;
+}
+
 void
 mrb_mruby_string_ext_gem_init(mrb_state* mrb)
 {
@@ -1400,6 +1423,7 @@ mrb_mruby_string_ext_gem_init(mrb_state* mrb)
   mrb_define_method_id(mrb, s, MRB_SYM_Q(valid_encoding), str_valid_enc_p,     MRB_ARGS_NONE());
   mrb_define_method_id(mrb, s, MRB_SYM_Q(ascii_only),     str_ascii_only_p,    MRB_ARGS_NONE());
   mrb_define_method_id(mrb, s, MRB_SYM(b),                str_b,               MRB_ARGS_NONE());
+  mrb_define_method_id(mrb, s, MRB_SYM(force_encoding),   str_force_encoding,  MRB_ARGS_REQ(1));
 
   mrb_define_method_id(mrb, s, MRB_SYM(__lines),          str_lines,           MRB_ARGS_NONE());
   mrb_define_method_id(mrb, s, MRB_SYM(__codepoints),     str_codepoints,      MRB_ARGS_NONE());