An image forming apparatus comprises a cartridge equipped with a toner reservoir, and an apparatus body. The cartridge is attached to the apparatus body when used and developer in the toner reservoir is used for image development. The cartridge comprises a storage unit which is provided inside the toner reservoir and capable of communicating with the apparatus body. The apparatus body comprises a communication unit which is capable of communicating with the storage unit and communication state between the storage unit and the communication unit varies depending on amount of residual developer in the toner reservoir. Thus there is provided a developer cartridge and an image forming apparatus surely capable of carrying out both residual-toner-amount detection and transmission of cartridge information with space saving design.