Disclosed are a developer cartridge designed to permit discrimination of whether the cartridge is used or unused, and an image forming apparatus using this cartridge. The developer cartridge comprises a cylindrical body whose rotational axis extends in a direction perpendicular to a developer carrying direction of the developing unit; a first developer storage room, provided at the cylindrical body, for storing an unused developer; an opening formed in the first developer storage room; a case having a cylinder for rotatably retaining the cylindrical body, the cylinder having an injection port provided in association with the opening of the first developer storage room; a second developer storage room, provided at a lower portion of the cylinder of the case, for storing a used developer from the developing unit; and setting means selectively movable to a first position indicating an unused state of the developer cartridge and a second position indicating a used state of the developer cartridge to detect if the developer cartridge is unused or used. The image forming apparatus comprising an endless latent carrier; means for forming an electrostatic latent image on the endless latent carrier; developing means for developing the electrostatic latent image on the endless latent carrier; a developer cartridge detachably attached to the developing means for exchanging developers, and having the same components as the above-described cartridge; drive means for driving the cylindrical body of the developer cartridge for developer exchange; detecting means for detecting the setting means of the developer cartridge to detect if the developer cartridge is unused or used; and a control circuit for controlling the drive means in accordance with a detection result from the detecting means.