An image data search system capable of easily and surely detecting when, where, and how image data has been processed in an image forming apparatus capable of setting various output forms. A storage server stores image data involved in each job executed by a copying machine. A history management server records information including at least information indicating an output form of image data in the job as history information. A search server searches for image data matching desired image data from the image data stored in the storage server, and detects history information corresponding to the image data stored in the storage server, from the history information recorded in the history management server. Further the search server changes a search method based on the information indicating the output form, which is recorded in the detected history information.