Toner cartridge, a devloping unit and image forming apparatus having the same

A developer cartridge, a developing unit and an image forming device including the same are provided to prevent the print error caused by the measurement error of the remaining quantity of a developer. A cartridge body(110) is detachably installed on a developing unit and has a developer discharge hole. A remaining developer quantity sensing unit(120) is closely installed to the developer discharge hole. A stirring unit transfers the developer filled in the cartridge body to the developer discharge hole. The remaining developer quantity sensing unit comprises the first light guide(121) and the second light guide(122).