In an image processing system, a controller of an image forming apparatus, when user identification information is entered, causes a communication device to transmit a transmission request for an encryption code to a mobile terminal device at which the user identification information has been set, and, when the communication device receives the encryption code, uses the encryption code to encrypt image data. A controller of the mobile terminal device, when a communication device receives the transmission request for the encryption code, converts a fingerprint of a user detected by a fingerprint sensor into the encryption code, and causes the communication device to transmit the encryption code to the image forming apparatus.