An image forming apparatus includes a main casing, a drum frame detachably attached to the main casing and including a photosensitive drum, a developing device supporting structure movable between a first position close to the photosensitive drum and a second position farther from the photosensitive drum, and a developing device supported by the developing device supporting structure and including a guided portion. The developing device supporting structure has a guide groove at an inner surface. The developing device is configured to be attached to or detached from the developing device supporting structure by the guided portion being guided along the guide groove. When the developing device supporting structure is in the first position, the developing device is attached to the developing device support structure, and when the developing device supporting structure is in the second position, the developing device is detachable from the developing device supporting structure.