An image forming apparatus includes a first developing cartridge and a first photosensitive cartridge. The first developing cartridge is removably mounted in the first photosensitive cartridge and includes a toner hopper, a supply roller and a developing roller. The first photosensitive cartridge includes a photosensitive drum, a charger provided at a position adjacent to the photosensitive drum, a first wall provided adjacent to the supply roller, the toner hopper, and the developing roller when the first developing cartridge is mounted in the photosensitive cartridge, the first wall including a first opening, and a second wall which faces the first wall and is provided adjacent to the charger, the second wall including a second opening on an opposite side of the charger from the photosensitive drum. A portion of a wall of the first developing cartridge projects through the first opening in the first wall of the first photosensitive cartridge.