A developer cartridge includes a cartridge body including a developer discharge port, a remaining developer detecting unit arranged in the proximity to the developer discharge port, and an agitating unit rotatably arranged inside the cartridge body to convey the developer accommodated in the cartridge body toward the developer discharge port. Since the optical sensor provided in the image forming apparatus body is capable of physically measuring an exact amount of remaining sensor of the developer cartridge, a printing error due to erroneous determination of the remaining developer amount can be prevented.