An image processing method is provided capable of improving a response to a user in a case where a series of images using volume data are displayed by volume rendering. When the images are displayed by animation in an oscillating manner with amplitude, the amplitude is gradually increased so as to spare time for image generation. That is, when the user specifies a parameter value, for example, a projection angle (initial value: θ=0°), for an object to be diagnosed, animation display of images C, D, and E of the object is performed with a small amplitude (θ=±5°) with the initial value as a center. During the animation display, an image B (θ=−10°) and an image F (θ=10°) are calculated in the background. After the calculation is completed, the amplitude is increased (θ=±10°) and animation display of the images B, C, D, E, and F is performed.