An image forming apparatus extracts, from image data, authentication data by which a user is identified, sends the extracted authentication data to a control terminal that recognizes a function available to the user identified by the extracted authentication data, and then receives, from the control terminal, available-function data indicative of a result of the recognition. Then, the image forming apparatus chooses, in accordance with the available-function data thus received, a function that is available in the image forming apparatus. This enables the image forming apparatus to validate or invalidate each function in accordance with a user on the basis of the extracted authentication data.