An image forming apparatus includes an image forming unit and a main body. The image forming unit is detachably mounted in a main body of the image forming apparatus and has at least a developing device for storing a predetermined developer for forming a desired image. A sensor circuit generates a detection signal upon detection of the state of the developer in the developing device. A connecting section outputs the detection signal to the main body. A transmitting section has a single portion to be connected and transmits, to the main body side, the detection signal and a signal representing a mounting state of the image forming unit, which is generated in accordance with a coupling state of the connecting section with respect to the single portion. A discriminating circuit has a single input terminal for receiving a signal from the single portion of the transmitting section, and discriminates the state of the developer in the developing device and a mounting state of the image forming unit with respect to the main body on the basis of the state of the signal input to the input terminal.