Abstract: A challenge is to provide an art to identify mice as individuals from captured images of one or more mice that are doing activity in a predetermined activity range. An image processing device includes an image acquisition unit, a part extraction unit, and an individual identification unit. The image acquisition unit acquires a captured video of a state in which one or more animals are doing activity in a predetermined activity range. An outline detection unit extracts the outlines of the bodies of the one or more animals from each of multiple unit images included in the video. The individual identification unit analyzes the multiple outlines detected from each of the multiple unit images by the outline detection unit, in a time-series manner and identifies the one or more animals as individuals in each of the unit images on the basis of the analysis results. Thus, the challenge is solved.