An image forming apparatus includes: a photosensitive drum; a developing unit; a casing; and a supporting member. The developing unit includes a developing agent bearing member. The developing agent bearing member has an axis extending in an axial direction and defines a first side and a second side opposite to the first side in the axial direction. The casing is configured to support the photosensitive drum and the developing unit. The casing has a sidewall at a position corresponding to the first side. The developing unit is detachable from and attachable to the casing along the axial direction. The supporting member is configured to guide a movement of the developing unit in the axial direction and protrudes outward from the sidewall.