A developing unit includes a body and a displacement member. The displacement member is to be held by the body in a first mode in a first mounting state, and is to be held by the body displaceably between the first mode and a second mode in a reversible fashion in a second mounting state. The first mounting state is a state in which a first toner cartridge is mounted on the developing unit. The second mounting state is a state in which a second toner cartridge is mounted on the developing unit.