A complex apparatus (image processing apparatus) requests the transmission of an identification number from a security box (data processing apparatus). The security box transmits the stored own identification number to the complex apparatus. When no response is received within a predetermined time after the request of transmission of the identification number, or alternatively when the identification number transmitted from the security box differs from the identification number of a security box stored in advance, the complex apparatus displays a warning screen indicating that the appropriate security box has been removed, on the displaying section of the operation panel, and then turns ON a notifying lamp.