In this paper, we develop a design method of nonlinear profile of gear ratio to utilize a DC servo motor effectively for a jumping robot. Because the larger ground force yields the higher kinetic energy of the robot body, the optimal gear ratio is obtained by the maximization of the ground force from statics point of view. Moreover, the varying gear ratio during the jump motion is obtained through a simulation which connects statics-based optimization and robot dynamics. A non-circular gear is synthesized which realizes the obtained optimal varying gear ratio. The effectiveness of the proposed method is evaluated by simulations.