By using information stored in a storage part provided in the image forming apparatus main body or the results of detection of an image density sensor or the like, it is judged whether or not a replaceable unit such as a toner cartridge has been replaced. Furthermore, from information stored in the storage part provided in the image forming apparatus main body, it is detected whether a replaceable unit such as a toner cartridge is genuine or other than genuine units. From these detection results, information on the amount used for developer of the replaced toner cartridge, stored in the storage part, is initialized.