In order to effectively generate an impact motion of a humanoid robot, a motion generation support system is developed. The system visualizes not only a designed motion but also experimented motion. Force and torque generated in the experiment are superimposed on the experimented motion. The visualized ZMP, GCoM, force, moment and difference between the designed motion and the experimental result will help motion designer to improve the designed impact motion. A nailing task is taken as an example of an impact motion. A motion of a humanoid robot to drive a nail into a wooden plate with a hammer is designed using this support system. The details of the motion generation support system and the nailing experiment performed by a humanoid robot are presented.