An image forming apparatus includes a main casing and a cartridge detachably mountable therein. The cartridge includes a photosensitive drum and a developing roller disposed to oppose each other. The main casing includes: a metal frame for supporting the cartridge; a lock member supported to the metal frame and movable about a first axis to allow or restrict movement of the photosensitive drum relative to the metal frame; and a contact-separation member supported to the metal frame and movable about a second axis to allow contact and separation of the developing roller relative to the photosensitive drum. The lock member and the contact-separation member are aligned in an orthogonal direction orthogonal to an axis of the photosensitive drum such that the first axis is not overlapped with the contact-separation member or the second axis is not overlapped with the lock member in an axial direction of the photosensitive drum.