1. An intermediate image generation method for 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 setting conditions for an opacity level to be allocated according to a pixel value, the method comprising the steps of: 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 becomes 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 the intermediate image by ray casting the three-dimensional image from an arbitrary viewpoint under the determined opacity level setting condition. such that the intermediate image appears to change substantially linearly with a linear change in the changing phase, wherein the determination of the opacity level is made by following Formula (1): 
 F α ( g )=(1.0− t (α))· F A ( g )+ t (α)· F B ( g )  (1) where g is a pixel value; F A (g) is a function representing the opacity level setting condition applied to the generation of the pseudo three-dimensional image at the start of the changing phase; F B (g) is a function representing the opacity level setting condition applied to the generation of the pseudo three-dimensional image at the end of the changing phase; α(0<α<1) represents the changing phase; t(α) is a monotonically increasing function, in which t(α) when F A (g)≦F B (g), while t(α)>u(α) when F A (g)>F B (g), u(α) =α; and F α (g) is a function representing the opacity level setting condition applied to the generation of the intermediate image.