A normal vector mapping unit maps a texture of the MIP-map level corresponding to a level of detail (LOD) of a polygon onto a polygon surface to generate a pseudo-normal vector on the polygon surface. A normal vector replacing unit refers to the vector quantization (VQ) table of the relevant MIP-map level and replaces the pseudo-normal vector on the polygon surface with a quantized reference normal vector. A brightness value acquiring unit refers to the color lookup table (CLUT) of the relevant MIP-map level, and acquires the brightness value corresponding to the reference normal vector on the polygon surface. The brightness value acquiring unit also interpolates brightness values between different MIP-map levels. A graphic processor combines the RGB value of each pixel of the polygon surface with the brightness value acquired by the brightness value acquiring unit to write the combined data into a frame buffer.