diff --git a/ext/groonga/rb-grn-utils.c b/ext/groonga/rb-grn-utils.c
index d9eb5e4e..adf21bac 100644
--- a/ext/groonga/rb-grn-utils.c
+++ b/ext/groonga/rb-grn-utils.c
@@ -171,6 +171,12 @@ rb_grn_equal_string (const char *string1, const char *string2)
     return strcmp(string1, string2) == 0;
 }
 
+VALUE
+rb_grn_convert_to_string (VALUE object)
+{
+    return rb_convert_type(object, RUBY_T_STRING, "String", "to_str");
+}
+
 VALUE
 rb_grn_convert_to_array (VALUE object)
 {
diff --git a/ext/groonga/rb-grn.h b/ext/groonga/rb-grn.h
index 6c065cfe..dda3443c 100644
--- a/ext/groonga/rb-grn.h
+++ b/ext/groonga/rb-grn.h
@@ -359,6 +359,7 @@ grn_bool       rb_grn_equal_option                  (VALUE option,
                                                      const char *key);
 grn_bool       rb_grn_equal_string                  (const char *string1,
                                                      const char *string2);
+VALUE          rb_grn_convert_to_string             (VALUE object);
 VALUE          rb_grn_convert_to_array              (VALUE object);
 VALUE          rb_grn_check_convert_to_string       (VALUE object);
 VALUE          rb_grn_check_convert_to_array        (VALUE object);