1. An image processing device comprising a storage unit, a CPU, and an output unit, wherein: the storage unit stores a plurality of pieces of moving image data; the CPU executes: a determination process of determining a similarity, with respect to a predetermined comparison viewpoint, of a motion of a subject in each of the plurality of pieces of moving image data stored in the storage unit; and a selection process of selecting a specific piece of moving image data from among the plurality of pieces of moving image data based on the similarity determined by the determination process; the output unit outputs information of a selection result by the selection process; the predetermined comparison viewpoint includes a plurality of comparison viewpoints from among (i) a first comparison viewpoint of a change in a state of the subject concerning various kinds of information to be analyzed in a series of motions of the subject at a plurality of motion timings, (ii) a second comparison viewpoint of the state of the subject concerning various kinds of information to be analyzed in a series of motions of the subject at a specific motion timing, and (iii) a third comparison viewpoint of the state of the subject in a still state concerning various kinds of information to be analyzed in the still state of the subject; and the determination process determines a final similarity of the motion of the subject based on an overall result of determinations regarding the similarity determined with respect to each of the plurality of comparison viewpoints.