PURPOSE: To control correctly an operating state of a process device in accordance with an optical characteristic of a toner of a developer of an installed developing device, by adjusting and controlling an operation of a control means in accordance with the optical characteristic of the toner in the developing device, by a discriminating signal.

CONSTITUTION: A reference voltage having a prescribed level is applied to each comparator 30, 31 and 32, respectively, in accordance with an optical characteristic of a toner in a developing device to be used. Accordingly, among those comparators 30, 31 and 32, an output of the comparator to which the reference voltage corresponding to the optical characteristic of the toner in the installed developing device becomes a signal for showing excess or shortage of density of a picture cuased by the optical characteristc of the toner in its developing device. In a selecting circuit 33, its signal is selected and supplied to an output terminal 34 by a discriminating signal from a discriminating signal generating part 29. On the other hand, a photoelectric detector 21 irradiates a light from a light emitting diode D1, to a toner image or a developer, converts it photoelectrically by photodetecting its reflected light by a phototransistor T1, and outputs it as a detecting signal.