1. An index data generation apparatus comprising: an acquisition device which acquires contents data that contains video data which includes at least a preset time length video data; an extraction device which extracts sub-image data, that is image data that aids contents of the main-image data that changes over time, from the main-image data that indicates said acquired video data; a first detection device which detects partial image data that indicates the areas of change in the images of said extracted sub-image data; a second detection device which detects the number of changes and points of change in each detected partial image data when at least two or more said partial image data are detected from one or two or more said sub-image data by said first detection device; a setting device which sets hierarchical levels for each partial image data based on the number of changes of said detected partial image data; and a generation device which generates index data that indicates a plurality of scenes in said contents data based on the hierarchical levels of each said detected partial-image data and points of change in each said detected partial-image data.