A cartridge type HDD unit 20 is mounted on the outside of a back plate panel 2 of an exterior cover rear portion in an image forming apparatus 1. An HDD main body is detachably attachable into a unit case 21. When the HDD main body is attached into the unit case 21, the HDD main body is substantially horizontally inserted from a sideways case opening 22. In the conventional art, there is the problem that the HDD main body is caused to fall carelessly in attachment working in which the HDD main body is mounted to a housing case such that the HDD main body drops into a housing case in a vertical direction. However the problem above described is eliminated, and the attachment working of the HDD main body can be performed easily and safely. With reference to security protection of information stored in the on-board mounted built-in HDD, the information stored in the built-in HDD can be transferred to and stored in the cartridge type HDD.