PURPOSE: To execute two controls effectively by the output of a common detecting device and to form a good image by controlling the quantity of light of a light source and the operation of a processing device of image forming by the output of a device that detects the quantity of reflected light of a exposed part.

CONSTITUTION: The quantity of reflected light from an original at a lamp 3 for exposure is detected by a photodiode 22. The output is inputted to a microcomputer 34, and an automatic picture density AE controlling voltage VB is supplied to a high voltage transformer 37 to change developing bias to be impressed to a developing device 8 and an AE function control is performed. Lamp driving high frequency voltage is formed by a converting oscillator 41 and current IL is supplied to the lamp 3 to control the quantity of light. Controlling signals are supplied to the transformer 37 and the oscillator 41 by a driver 42 according to the output of the oscillator 41. The quantity of light and the AE function are controlled by the common device, the light source is made stable and the exposure is improved. Thus, a good image according to the density of an original can be formed.