A developing device mountable in a main body of an image forming device includes: a frame, a developer cartridge attachable to the frame, and a maintaining member. The developer cartridge includes: a casing having an opening; and a shutter movable between an open position and a closed position to open and close the opening. The developer cartridge is movable between a first position to be attached to the frame and mountable in the main body and a second position to be partially detached from the frame and incapable of being mounted in the main body. The shutter moves from the closed position to the open position while the developer cartridge moves from the second position to the first position. The maintaining member maintains the shutter at the closed position when the developer cartridge moves from the second position to the first position.