PURPOSE: To detect the fault of electric charge system easily and surely by comparing the grid voltage, currents, etc. between corona electrodes and a photoreceptor with set value to decide the absence or presence of any fault.

CONSTITUTION: Photoreceptor is charged to a prescribed potential with a scorotron charger 18 provided with corona wires 30 applied with high voltages from a high voltage generating circuit 33, grid wires 31 applied with the constant voltage of a constant voltage circuit 34, etc. The constant voltage to be applied to the wires 31 is compared with a comparator CMP 1, COMP 2 in which upper and lower limit voltages are set. On the other hand, the voltage at the other terminal of the circuit 34 corresponding to the currents flowing in the wires 31 is compared with a prescribed set value in a comparator CMP 3. The compared outputs of the comparators CMP 1WCOMP 3 and the abnormal output of the circuit 33, etc. are processed with the AND circuits AN 1WAN 4 of a fault decision circuit 35, an OR circuit OR 1, etc. to detect automatically the various faults of the charge systems. Thus, the faults of the charge systems are detected easily and securely without recording, etc.