PURPOSE: To improve the reliability of a detection circuit which is utilized to control the replenishment of toner, to simplify the constitution of a circuit and to reduce cost by adjusting the light emission energizing quantity of a detecting means by controlling the duty of a pulse and adjusting detection sensitivity.

CONSTITUTION: Every time copying is executed for ten copies, a sheet number counter is cleared and a P sensor pattern image and the output level of a P sensor for the surface of a photosensitive body is read. In the case of deciding that toner concentration is sparse, as the result, a toner replenishment flag is controlled to be on and the replenishment of toner is executed every time a copying process for one sheet is executed. When the P sensor performs read, a pulse duty value stored in a memory at the time of initialization is read out and the pulse is generated from a port COO by using this value and the current value for the light emitting diode of the P sensor is decided. Therefore, even when the characteristic of the P sensor is changed, detecting characteristic is kept constant by readjusting the current value.