1. An image processing device comprising: an image obtaining unit for obtaining a first image obtained by imaging a subject with a first modality and a second image obtained by imaging the subject with a second modality, the second modality being different from the modality with which the first image is obtained; an image deformation amount estimation unit for estimating an image deformation amount for deforming the first image to provide a deformed first image that is similar to the second image by deforming the first image and evaluating similarity between the deformed first image and the second image with an evaluation function, the evaluation function evaluating correlation between distributions of pixel values of the deformed first image and corresponding pixel values of the second image; and an image generation unit for generating a deformed image of the first image based on the estimated image deformation amount, wherein the image deformation amount estimation unit comprises a probability information obtaining unit for obtaining probability information indicating a probability of each combination of corresponding pixel values of the first image and the second image, and the evaluation function includes a term representing a measure of correlation between a pixel value of the deformed first image and a corresponding pixel value of the second image, wherein the term evaluates the correlation based on the obtained probability information.