In order to exert large force on an environment, it is effective to apply impulsive force. We describe the motions that perform tasks by applying impulsive force as “impact motion.” The objective of an impact motion is to exert large force on an environment, however if the impulsive force is too large, the robot may fall down due to the reaction force. This paper presents an optimization scheme to generate impact motions for humanoid robots. The advantage of the proposed scheme is that impulsive force exerted on a target by a humanoid robot's whole body is maximized while guaranteeing the stability. A punching motion is generated by the scheme as an example and evaluated by performing simulations.