PURPOSE: To control the concentration of toner always at a constant value even in a humidity environment by arranging a humidity detector near a two- component developer.

CONSTITUTION: In the relation between humidity and the inductance of a developer, the inductance is increased in accordance with the increase of humidity. Even if the toner concentration is fixed, the characteristic of the humidity detector kept at a high temperature reduce the sensor resistance like exponential function in accordance with the rise of humidity. The output of the humidity detector 8 can be extracted as voltage change by an AC regulated power supply 9. In addition, an output having a negative characteristics to humidity by a logarithmic amplifier 10 and a rectifying circuit 11. Since the humidity characteristic of the humidity detector 1 is positive, arithmetic weighted in accordance with respective variables is executed by an arithmetic circuit 12, so that the humidity change of a detecting level of the density detector 1 can be canceled.