A toner cartridge stored in a developer casing includes a biasing force generating device, an actuating rod that switches the biasing device between an active state and a non-active state by one movement of a release member. A developer casing swings by the movement of the actuating rod and presses a developing roller with appropriate pressure to a photosensitive drum. There is a lock member that is restrained from moving by the movement of the actuating rod. The structure recited is capable of inserting and fixing the developer casing to an image forming device. Also, by one restore movement of the release member, the photosensitive drum and the developing roller are separated and the lock is released, thereby enabling removal/exchange of the developer casing.