When generating a pseudo three-dimensional image by performing volume rendering on a three-dimensional image using an opacity curve that defines the relationship between pixel value and opacity, identifying a whole region representing a predetermined target object from the three-dimensional image, setting a base opacity curve to the identified whole region, obtaining, with respect to each of at least some pixels in the identified whole region, a representative value in an adjacent region of a pixel concerned, and setting an opacity curve obtained by modifying the base opacity curve using the obtained representative value as the opacity curve to be applied to the pixel concerned in the volume rendering.
