In most RPG games, gameplay tends to be monotonous due to fixed character skills. This paper proposes a RPG game implementation technique which allows players' skill customization for their own gameplay. Players are allowed to customize their preferred game skills at the beginning of gameplay. We also propose a technique to guarantee game balancing by prohibiting certain skills from dominating gameplay. We implemented the proposed skill customization method together with a simple RPG game testbed. We tested a variety of player-defined skills and confirmed the method provides good game balancing during gameplay. Due to the voluntary and impromptu skill customization, a wide variety of strategy is possible even at the single game stage, which reduces boredom at the repetitive gameplay.