An image processing apparatus includes a current image acquisition device which acquires a current image of a subject having a structure having a periodic movement, a past image acquisition device which acquires multiple past images of the subject such that the past images captured for over one or more periods of the periodic movement are acquired, and circuitry which searches multiple feature points on each past image and the current image, associates the feature points on the current image and the feature points on each of the past images, calculates, for each of the past images, a degree of similarity between the feature points on each of the past images and the feature points on the current image based on association, and identifies to which one of the past images the current image corresponds such that at which phase of the periodic movement the current image is positioned is estimated.