1. An image processing apparatus comprising: a CPU configured to: obtain an acquired image at a first time point as a standard image from time-series acquired image data obtained by an image acquisition unit acquiring a tracking subject and select a plurality of partial image regions including an image region of the tracking subject in the standard image, each of the plurality of partial image regions being separately spaced from the other of the plurality of partial image regions; obtain an acquired image at a second time point that is different from the first time point as a reference image and detect a movement of the tracking subject between the standard image and the reference image based on a difference between a state of pixel values of the partial image regions of the standard image and a state of pixel values of partial image regions of the reference image; set a center of each of the plurality of partial image regions as a standard point used as a standard for relative positions of the partial image regions; and obtain the state of the pixel values of the partial image regions of the reference image with the relative positions being unchanged, the relative positions being the relative positions of the partial image regions with respect to the standard point, and calculate the difference between the obtained state of the pixel values of the partial image regions and the state of the pixel values of the partial image regions of the standard image to calculate a position of the standard point in the reference image.