In a system for providing an illegal use research service for image data, the image data received from a terminal apparatus is registered as a research subject, a research condition for research whether or not the image data being registered is illegally used by a Web site on the network is set, image data that are identical or similar to the image data being the research subject from a web site on the network are searched for based on a feature amount and the research condition, and a search result is informed as a research report to the research client using said terminal apparatus.