An image processing method for registering first and second images including the same subject is provided. The method includes the repeating the following steps multiple times: a first step including determining feature vectors related to gradients of pixels values at coordinates on the first and second images for each of the coordinates, a second step including calculating for all mutually corresponding coordinates on the first and second images, correlation values each equivalent to a value obtained by an N-th (where N is a natural number) power of an absolute value of an inner product of the feature vectors at the coordinates and determining an evaluation value including an integrated value of the correlation values calculated for each of the coordinates, and a third step including changing the second image in such that the evaluation value becomes larger.