Abstract: A joint having an OLDHAM state coupling at a leading edge thereof is held movably in a direction which is parallel to a joint axis at a full-color printer body. A coupling gear is substantially connected to a developing roller disposed in each mono-color developing unit, which makes up a revolving type developing device. At the time of developing by one of the mono-color developing units, the joint is capable of moving in parallel to the axis of the joint in the state of being rotated toward the coupling. A projection is further provided at the leading edge of the coupling, and a claw is further provided at the end of the coupling gear. Thereby, smooth engagement between the coupling gear and the coupling is made by the help of the OLDHAM state coupling, the projection and the claw. As a result, the driving force is transmitted from the joint to the developing roller disposed in the developing unit, so that the developing roller can rotate at a predetermined speed and direction.