To provide design methods of a golf club, a golf ball, and a combination of golf club-golf ball, which satisfy desired optimal conditions of a characteristic value of the golf club or a characteristic value of behavior of the golf ball immediately after impact.

A swing rhythm coefficient is a ratio between a first time from starting of back swing to reaching to a top state, and a second time from the top state to starting of down swing to a shot of the golf ball, which is performed by a golfer during a golf swing. The swing rhythm coefficient is used for determining an optimal natural frequency of the golf club when head speed of the golf club is approximately maximum at the time of shooting golf ball. Swing simulation is performed for selected golf clubs based on the natural frequency value for obtaining an optimization design plan of the golf club or the golf club-golf ball.