Developer replenishment unit and image formation apparatus

1. Field of the Invention An image formation apparatus includes a developing unit for developing latent image, and a reserve tank that temporarily stores developer ejected from a first cartridge and a second cartridge and that discharges the developer to the developing unit. The reserve tank is provided with a storage amount sensor for sensing storage amount of the developer in the first reserve tank. The image formation apparatus further includes a controller for controlling ejection of developer from at least the first cartridge and the second cartridge into the first reserve tank in accordance with output from the storage amount sensor.