A toner replenishing device that provides a container and a device having a high toner volumetric efficiency and a low cost, suppresses vibration during replenishment to a minimum, and stores recovered toner without impairing the volumetric efficiency of unused toner is constituted by a developer replenishing device which is provided with a developer container storing a developer used in an image forming apparatus and replenishes a developing device of the image forming apparatus with the developer in the developer container. The developer container is rectangular in a horizontal direction, and comprises an outside container storing unused developer and a deformable inside container enveloped by the outside container. The developer replenishing device comprises a developer discharging mouthpiece portion for discharging the developer from the interior of the container, which is provided in a lengthwise direction end portion of the outside container, an air supply nozzle, provided in an opposite side end portion to the developer discharging mouthpiece portion in the lengthwise direction, for introducing air into the inside container, and an air pump for supplying air to the supply nozzle.