A system that provides a compact and functional image forming device that can securely supply a driving force to the developing cartridge is described. A developing cartridge is described that can be mounted to the image forming device in an insertable/removable manner. A passive coupling gear and cylinder externally project in the width direction of the collar member allowing secure supply of a driving force through connection of the passive coupling gear to the coupling input shaft. In addition, the amount of movement of the coupling input shaft to connect with the passive coupling gear can be minimized, thereby allowing for a miniaturization of the color laser printer. Furthermore, in addition to the original function, the function of being guided by the left guiding groove for the passive coupling gear and the function of being guided by the right groove for the cylinder can be added.