A data processing method, which is provided in an image processing system in which a server and a data processor communicate with each other to perform a predetermined data process, includes a step of extracting a feature amount of electronic data to be output from a printer, a step of transmitting original specifying information including the extracted feature amount and a user ID for discriminating a print requester of the electronic data, to the server, a step of controlling generation of print data to be output to the printer, based on original certification information notified by the server, and a step of causing the printer to print the generated print data and information indicating that the print data corresponds to an original. By this method, a large storage capacity is not required to certify one data, and to keep a secret of the stored electronic data is easy.