A printer for printing character information on an optical disk using an ink cartridge. The printer comprises a printer cover for opening/closing a cartridge receiving section into which the ink cartridge will be loaded. When the ink cartridge is loaded into the cartridge receiving section, the cover is closed and then the optical disk is inserted into the printer. Then, printing is started. The cover is engaged with the printer body in a closed state. The cover is disengaged by operating an operation button. When the optical disk is inserted into the printer body, an inlet cover is pushed by the optical disk so as to be upstanding. This causes a lock member to protrude so as to engage with the operation button, thereby locking the operation button and hence rendering the cover unopenable.