A developer cartridge which is rotatably mounted in the developing unit of an image forming apparatus, and supplies a developer to the developing unit while rotating includes a cylindrical cartridge main body having a developer outlet hole in the outer surface near one end, and a ring-like cartridge-side shutter which is fitted on the outer surface near one end of the cylindrical cartridge main body to be movable along the rotating shaft of the cylindrical cartridge main body between a position where the developer outlet hole is opened and a position where the developer outlet hole is closed. The developing unit has a guide for inserting the developer cartridge and a driving unit for rotating the developer cartridge, and the guide has a main body-side shutter with a hole. When the developer cartridge is mounted, the cartridge-side shutter moves from the position where the developer outlet hole is closed to the position where the developer outlet hole is opened, and the developer outlet hole aligns itself with the hole of the main body-side shutter. The cylindrical cartridge main body and the main body-side shutter integrally rotate in align the developer outlet hole and the hole of the main body-side shutter with a developer replenishment port formed below the main body-side shutter. Every time this happens, the developer in the developer cartridge is supplied from the developer replenishment port to the developing unit via the developer outlet hole and the hole of the main body-side shutter.