An .alpha.-value .alpha.P1 corresponding to a Z-value of each pixel in an original image and an .alpha.-nonuniformity setting .alpha.-value a UP which is a nonuniform .alpha.-value in a virtual plane are synthesized into an .alpha.-value .alpha.P2. The color ICP of the original image and the fog color CP are synthesized into a fog image, based on the .alpha.-value .alpha.P2. A plurality of .alpha.-nonuniformity setting .alpha.-values provided for the virtual planes different in nonuniformity pattern are synthesized. The nonuniformity patterns are changed depending on the virtual camera information and time information. An .alpha.-value used to synthesize the .alpha.-nonuniformity setting .alpha.-values is set based on a Z-value of each pixel in the original image. A Z-value of each pixel in the original image is used as an index number of a lookup table and the index color texture mapping is applied to a virtual object to convert the Z-value into an .alpha.-value.