An image forming apparatus capable of preventing the return of a power supply switch from being forgotten and eliminating the troubles due to the forgetting of this return and having a sheet processing device attached to the main body thereof. The sheet processing device includes a driving source for actuating the sheet processing device, a power supply for supplying the power to the driving source and a switch for the switching of the power supply to ON/OFF, wherein the switch becomes switchable to ON condition on detaching the outer cover of the sheet processing device and is switched to OFF condition on attaching the outer cover.