A monitor target shooting information generation unit extracts, from each of a plurality of images, as information to be used for estimating the identify of a monitor target, monitor target identification information that is identification information of the monitor target, and the monitor target shooting information generation unit then generates multiple pieces of monitor target shooting information each including both the extracted monitor target identification information and a shooting time at which the monitor target was shot. An appearance history generation unit generates, from the generated multiple pieces of monitor target shooting information, an appearance history of the monitor target that has been estimated to be identical. A determination unit determines the monitor target the appearance history of which matches a specified rule.