When generating an intermediate image which is an image in the middle of changing from one to the other of two pseudo three-dimensional images generated by ray casting a three-dimensional image from an arbitrary viewpoint under two different opacity level setting conditions, determining an opacity level setting condition by determining an opacity level to be allocated according to each pixel value such that a variation in the opacity level for a change in a changing phase of the intermediate image become smaller as the changing phase approaches closer to a pseudo three-dimensional image generated under either one of the two different opacity level setting conditions in which a smaller value is allocated as the opacity level to be allocated according to each pixel value, and generating an intermediate image by ray casting the three-dimensional image from an arbitrary viewpoint under the determined opacity level setting condition.