1. An image processing device that synthesizes a plurality of images obtained by photographing an object that moves at a predetermined speed to thereby obtain a synthesized image, comprising: a detection unit configured to divide each of the plurality of images into a plurality of blocks, and detect a motion vector from each of the blocks; a selection unit configured to estimate an object maximum movement amount indicative of a maximum amount of a movement amount of the object between the plurality of images based on a focal length and an exposure interval which are applied when each of the plurality of images is obtained, and to select motion vectors having a scalar amount smaller than the object maximum movement amount; and a synthesis unit configured to perform synthesis processing by aligning the plurality of images based on motion vectors selected by said selection unit to thereby obtain the synthesized image.