To make easily and exactly judgeable whether or not a detachable unit including an image formation auxiliary means and a mechanism control auxiliary means is perfectly mounted onto a main body, to make detectable simultaneously a power failure and a failure in closing an inspection door, and to suppress influence on a radiated noise by operating only one prescribed sensor deduced from the power feeding timing signal among the toner density detecting sensors of four developing device units by switching the power feeding, and shortening an output signal to each sensor.

The detecting device is provided with a means to monitor a power supply voltage supplied to the detachable image forming unit, and a means to monitor a driver voltage of the image formation auxiliary means formed in the unit, and judges the existence of the unit from a monitoring result of driving state and voltage of the image formation auxiliary means.