1. A system for compressing a motion picture generated by projecting objects onto a screen, the objects moving relatively to the screen in a three-dimensional (3D) space, the system comprising: a motion calculating component which calculates a motion vector of the object within the screen by projecting onto the screen a motion of the object in the 3D space between a time corresponding to a target frame to be compressed and a time corresponding to another frame; and a compressing component which compresses the compression target frame by motion compensation based on the motion vector, wherein the compressing component compresses a difference image obtained by subtracting an image in a region in the another frame from an image in a region including the object projected onto the compression target frame, the another frame being specified by the motion vector associated with the region including the object; and, an adjustment component which adjusts the motion vector so as to allow the image in the region including the object projected onto the compression target frame and the image in the region specified by the motion vector in the another frame to further coincide with each other, said adjustment component comparing the image in the region including the object projected onto the compression target frame with images in each region obtained by shifting, by a value within a predetermined range in number of pixels, the region specified by the motion vector in the another frame, and changes the motion vector so as to specify, among the shifted regions on the another frame, one having an image coinciding the most.