In the second layer, one node (representative) selected from the plurality of groups in the third layer is arranged and linked with the corresponding group of the third layer. The plurality of nodes arranged in the second layer are grouped such that those having a similarity (similarity between the feature values shown in FIG. 5) equal to or higher than a predetermined level are grouped together.

The criteria (second threshold value) for similarities for grouping in the third layer is higher than the criteria (first threshold value) of similarity for grouping in the second layer. The second threshold value serves as the “predetermined level” in the “processing for determining whether or not the outer appearance feature value of each person detected from the frame to be processed is similar by the predetermined level or more to that of any of persons detected in previously processed frames”.

In the first layer, one node (representative) selected from the plurality of groups in the second layer is arranged and linked with the corresponding group of the second layer.

The detection ID of a person detected in a frame processed before the frame to be processed is indexed in this manner.

Next, using this index, the “processing for determining whether or not the outer appearance feature value of each person detected from the frame to be processed is similar by the predetermined level or more to that of any of persons detected in previously processed frames” will be described.