An image processing device including: one or more processors comprising hardware, wherein the one or more processors are configured to: calculate an estimated movement amount of a subject in each image of a plurality of images; perform, based on the estimated movement amounts, one of: select and output an image that is most recently captured among the plurality of images; and select a reference image from the plurality of images based on the estimated movement amounts of the subject in the plurality of images; in response to selecting the reference image, determine a gain of the reference image; and perform, based on the gain, one of: select and output the reference image; and a synthesis process including: select a synthesis target image from the plurality of images; and generate and output a synthesized image by synthesizing the synthesis target image and the reference image.