1. An articulated object region detection apparatus which detects a region having an articulated motion, using trajectories which correspond, in one-to-one relationship, to regions included in a moving object in a video, said articulated object region detection apparatus comprising: a subclass classification unit configured to classify trajectories into subclasses each of which is a group of similar trajectories, each of the trajectories being a trajectory of a motion of a block and being over pictures included in the video, the block including one or more pixels each of which constitutes the pictures; a distance calculating unit configured to calculate, for each of the subclasses, a point-to-point distance and a geodetic distance, the point-to-point distance being a distance directly connecting a first trajectory that belongs to the subclass and a second trajectory that belongs to another one of the subclasses, the geodetic distance being a distance of a path that passes through, as a relay point, a trajectory other than the first trajectory and the second trajectory, to reach the second trajectory from the first trajectory; and a region detection unit configured to detect, as a region having the articulated motion, two subclasses that include trajectories corresponding to two regions connected via a same articulation and including the articulated motion, based on a temporal change in the point-to-point distance and a temporal change in the geodetic distance between two given subclasses among the subclasses.