In an image processing method for generating a pseudo three-dimensional image by a volume rendering method in which a pixel value of each pixel on a projection plane on which a three-dimensional image illuminated by an arbitrary light source is projected is determined using a brightness value at each examination point, which is a point on the three-dimensional image sampled along each of a plurality of visual lines connecting an arbitrary viewpoint and each pixel on the projection plane, an illuminance level at each examination point is calculated based on an opacity level at each calculation point, which is a point on the three-dimensional image sampled along each of a plurality of light rays connecting the light source and each examination point, and the brightness value at each examination point is determined based on the calculated illuminance level.