To provide the real time rendering of an object whose materials are provided with under surface scattering such as the skin of a human body, wax, and marble.

A curvature index is read about each of the vertexes of the polygon mesh of an object from a memory (102), and the curvature index is calculated about each of the indexes of the polygon mesh of the object, and the curvature index of a polygon is interpolated during rasterize processing. In this case, a "curvature index" is the maximum main curvature direction unit vector t1 and two parameters K and M. K is the half value of the total of the square of main curvature and M is the half value of the difference of the square of the main curvature. A process for interpolating the curvature index of the polygon is executed by linearly interpolating a triangle as the primitive of the polygon.