An image processing apparatus which makes it possible not only to improve reusability of data in another apparatus, but also to increase the speed of data output. If a process is requested to be executed on the image data and the file data contains data usable by the image processing apparatus in addition to the image data, the apparatus executes the requested process using the usable data, whereas if the file data does not contain data usable by the image processing apparatus in addition to the image data, the apparatus executes the requested process using the image data. Further, the apparatus adds data generated during execution of the requested process using the image data to the file data, as data usable by the apparatus.