An image forming apparatus has a first opening on a side surface of the casing, first and second frames, a cartridge detachably attached through the first opening, and light receiver and light emitter respectively provided to the first and second frames. The cartridge shields the light from the light emitted when attached to an attachment position. The first and second frames face across the attachment position. The light reaches the light receiver when the cartridge is detached from the casing. A door is movably secured to the casing between closing and opening positions, and a second opening allowing a sheet to pass through is formed on the door. External light entering through the second opening and directed toward the light receiver is shielded by a light shielding part provided to the first frame, which also serves as a guiding part to guide attachment of the cartridge.