An articulated object region detection apparatus which detects regions 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; the articulated object region detection apparatus being characterized by further comprising: a distance calculating unit configured to calculate, for each of the subclasses, point-to-point distances and geodetic distances, each point-to-point distance being an Euclidean distance directly connecting a first trajectory that belongs to the subclass and a second trajectory that belongs to another one of the subclasses, each geodetic distance being a distance of a shortest 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, the distance calculating unit calculating each geodetic distance by non-linearizing the Euclidean distances between the trajectories by replacing, with an infinite value, Euclidean distances larger than a predetermined threshold, or by replacing, with an infinite value, Euclidean distances larger than the Nk-th Euclidean distance from the smallest among the Euclidean distances, where Nk is a threshold specified in advance; 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 that indicates the articulated motion, based on a temporal change in a representative value of the point-to-point distances and a temporal change in a representative value of the geodetic distances between the two subclasses, each of the temporal change in the point-to-point distance and the temporal change in the geodetic distance being a value based on a time derivative of the respective representative value, wherein said region detection unit is configured to detect, as the region having an articulated motion, a subclass that is one of two subclasses between which the temporal change in the point-to-point distance is larger than a predetermined threshold of the point-to-point distance and the temporal change in the geodetic distance is equal to or smaller than a predetermined threshold of the geodetic distance. 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; the articulated object region detection apparatus being characterized by further comprising: a distance calculating unit configured to calculate, for each of the subclasses, point-to-point distances and geodetic distances, each point-to-point distance being an Euclidean distance directly connecting a first trajectory that belongs to the subclass and a second trajectory that belongs to another one of the subclasses, each geodetic distance being a distance of a shortest 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, the distance calculating unit calculating each geodetic distance by non-linearizing the Euclidean distances between the trajectories by replacing, with an infinite value, Euclidean distances larger than a predetermined threshold, or by replacing, with an infinite value, Euclidean distances larger than the Nk-th Euclidean distance from the smallest among the Euclidean distances, where Nk is a threshold specified in advance; 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 that indicates the articulated motion, based on a temporal change in a representative value of the point-to-point distances and a temporal change in a representative value of the geodetic distances between the two subclasses, each of the temporal change in the point-to-point distance and the temporal change in the geodetic distance being a value based on a time derivative of the respective representative value, wherein said region detection unit is configured to detect, as the region having an articulated motion, a subclass that is one of two subclasses between which the temporal change in the point-to-point distance is larger than a predetermined threshold of the point-to-point distance and the temporal change in the geodetic distance is equal to or smaller than a predetermined threshold of the geodetic distance.