Using two or more developer replenishing units, each of which has a first storage for storing identification information related to developer and a second storage for storing history information related to the developer, if the identification information read out from each developing replenishing unit accords with unique information stored in an image forming main body, the history information is read out from the developing replenishing unit and analyzed to judge whether the utilization amount of the developing replenishing unit is at the end of its life. If the developing replenishing unit is not at the end of its life, the discharge amount of the developer replenished from the developer replenishing unit is controlled according to the history information and image output results formed by means of the image forming main body.