PURPOSE: To attain a copy image having a uniform density even if the destaticizing light quantity of a fluorescent lamp is changed, by detecting the destaticizing light quantity of the fluorescent lamp with a light quantity sensor to control a voltage impressed to an electrostatic charger corotron.

CONSTITUTION: A light quantity sensor 6 which detects the quantity of light of a fluorescent lamp 1, a function amplifier 7 which outputs a preliminarily set signal in accordance with the output signal of the light quantity sensor 6 when the output signal is supplied from the light quantity sensor 6, a high voltage power source 8 which changes the value of the voltage impressed to an electrostatic charging corotron 2 by the output of the function amplifier 7, etc. are provided. Though the fluorescent lamp 1 destaticizes the residual electric charge on the surface of a photoreceptor 5, the destaticizing light quantity is easily changed by the change of the ambient temperature, and consequently, the developing potential is changed. Therefore, the voltage impressed to the electrostatic charging corotron 2 is changed to attain a constant dark developing potential. Thus, the dark developing potential is kept constant to obtain the copy image having a uniform density.