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 developing roller and a guided portion. The developing device supporting structure has a guide groove at an inner surface. The guided portion is disposed in the guide groove to guide the developing device between a third position close to the photosensitive drum and a fourth position farther from the photosensitive drum. When the developing device supporting structure is in the first position, the developing device is in the third position, and when the developing device supporting structure is in the second position, the developing device is in the fourth position.