An image processing device that is capable of synthesizing a plurality of images by accurately correcting an image shift even in a photographing scene with a lot of external disturbances, such as noise. An image processing device synthesizes a plurality of images obtained by photographing an object that moves at a predetermined speed to thereby obtain a synthesized image. Each of the plurality of images is divided into a plurality of blocks, and a motion vector is detected from each of the blocks. Selection from the motion vectors is performed based on a focal length and an exposure interval applied when the plurality of images are obtained. Synthesis processing is performed by aligning the plurality of images based on the selected motion vectors, whereby the synthesized image is obtained.