An image forming apparatus according to one aspect comprises: a main unit; and a developer cartridge detachably attached to the main unit. The developing cartridge comprises: a rotation body having a rotational axis and rotatable in a rotation direction; and one rotational arm of a first rotational arm or two rotational arms of the first rotational arm and a second rotational arm which are rotatable around the rotational axis. The main unit comprises: a swingable detecting arm; an elastic member that urges the detecting arm toward a neutral position. When the developer cartridge is attached to the main unit, the first rotational arm contacts with the detecting arm to swing in a first swing direction from the neutral position. When the rotation body rotates by a predetermined rotation amount, the second rotational arm contacts with the detecting arm in a second direction from the neutral position.