This invention provides an image processing device capable of appropriately generating a synthetic image with an enlarged dynamic range even when a measuring article is moving. A work to be a reference (reference work) is imaged a plurality of times under an exposure condition same as that for generating a synthesized image, and specified regions are registered as reference images from a plurality of input images acquired in the respective imaging processes. In an operation mode, a plurality of input images is acquired by imaging a work a plurality of times under a plurality of exposure conditions, and search processes based on the reference images are executed on the respective input images. A partial image having a predetermined size is extracted from each of the plurality of input images based on each specified position, and an image synthesizing process is executed on the plurality of extracted partial images to generate a synthesized image.