A laser printer having a self-diagnostic function includes a first sensing unit that changes in detection status when one of a process cartridge and an inspection cartridge is mounted therein, and a second sensing unit that changes in detection status when at least the other of the process cartridge and inspection cartridge is mounted therein. A control unit of the laser printer identifies the type of cartridge mounted in the laser printer based on detection results from the first and second sensing units and switches the operating mode of the laser printer from a normal mode for performing image-forming operations to a self-diagnostic mode for diagnosing the status of its components when the control unit determines that the inspection cartridge is mounted in the laser printer.