Texture data filtered according to each of different reduction ratio are stored in a texture buffer. A texture mapping apparatus (an lod calculating apparatus) calculates an lod (Level Of Detail) which represents a reduction ratio of each pixel of a polygon. The calculation does not include a divisional calculation. In other words, the lod calculating apparatus does not need many multipliers, as compared with a case in which an operation including a divisional calculation, thereby enabling the down-sizing of the lod calculating apparatus.