An image generation system generates an image of an object space viewed from a given viewpoint. When a contact event in which a first model object comes in contact with a second model object in the object space has occurred, height information of grid points corresponding to a contact plane of the first model object is fetched by referring to height map data in which height information of a surface of the first model object with respect to a given reference plane set for the first model object is set in a grid pattern, and vertices of a contact plane of the second model object arm moved based on the fetched height information of the grid points.