1. An image processing apparatus comprising: a storage unit which stores an X-ray image for each frame which is acquired by applying X-rays to a subject to be examined while an X-ray tube is moved along a predetermined scan track and a different filter coefficient for each scan track, each frame of each X-ray image, and each pixel position of a detector; a filter processing unit which determines a filter coefficient for each pixel of each X-ray image on the basis of a combination of the predetermined scan track, the frame of the X-ray image, and the pixel position of the detector, and executes filter processing for an X-ray image or a processed image thereof for said each frame by using the determined filter coefficient; and a tomosynthesis image generating unit which generates a tomosynthesis image by using an X-ray image for said each frame after filter processing.