To obtain an image generation system and an information storage medium which can actualize the real shape deformation of a model object with a small number of motion bones.

The position and rotational angles of motion bones BM11 and BM12 are found from motion data and the position and rotational angle of an auxiliary bone BS7 (assisting the deformation of the model object) are so found that it is arranged at a different position from the BM11. The model object is deformed following up the BM11, BM12, and BS7. The auxiliary bone BS7 is arranged facing a shoulder JS. Even when the motion bone BM11 rotates on an X axis, the auxiliary bone BS7 is not rotated. The coordinate matrix of the auxiliary bone BS7 is so found that the auxiliary bone BS7 faces the shoulder JS and is arranged at a different position from the motion bone BM11 and in different motion. The flesh nearby the collarbone and shoulder is made to follow up the auxiliary bone BS7 instead of the motion bone BM11.