1. A motion blur image rendering method for a three-dimensional computer graphics, the method comprising: generating first information concerning three vertexes in a two-dimensional triangle image as an object at two points of time to obtain second information concerning six vertexes; dividing into three tetrahedrons a triangular prism formed by the six vertexes in a three-dimensional space defined by a pixel coordinate axis of the two-dimensional triangle image and a time axis; and subjecting each of the three tetrahedrons to linear processing to obtain rendering information for a motion blur image.