A developing unit includes a developer cartridge containing developer and a developing cartridge with a developing cartridge body comprising a mounting portion and a cover member. The mounting portion accepts the developer cartridge for replenishing developer consumed by the developing cartridge. The cover member closes an opening of the mounting portion when the developer cartridge is not inserted into the mounting portion. When a developing unit is initially constructed, printing is performed using developer contained in the developing unit. A developer cartridge containing developer can be inserted into the developing unit, allowing the supply of developer in the developing unit to be replenished.