1. An image processing apparatus comprising: an acquisition unit configured to acquire a pixel value range for each of a plurality of partial images captured for a plurality of partial imaging ranges which an imaging region of a subject is divided into, and a value indicating pixel value distribution of an overlapping area in each of the plurality of partial images; a calculation unit configured to calculate for each partial image a shift amount for minimizing a difference between the values indicating pixel value distribution of the overlapping areas, and setting each of the pixel value range within a predetermined range; and a correction unit configured to shift pixel values by the shift amount for each partial image.