To perform shading corresponding to the raggedness of a polygon surface by correcting the image data of a display pixel according to the inner product of a normal vector and a ray vector.

A display counter 71 in an image data correcting part 70 generates a coordinate D71 of the display pixel at the timing of a video lock CLK synchronously with prescribed vertical and horizontal synchronizing signals and generates a coordinate D72 of surrounding pixels adjacent to the coordinate D71 through an adjacent coordinate generating part 72. Then, both the coordinates D71 and D72 and Z value data D60 corresponding to these coordinates are supplied to a unit normal vector calculating part 73 and the unit normal vector of the display pixel is calculated. The inner product value of both the vectors is found from this unit normal vector and a unit light ray vector generated by a unit light ray vector calculating part 75 by an inner product calculating part 77, and based on the inner product value, color data D50 are corrected by a color correcting part 78.