PURPOSE: To adjust the oscillation frequency of the oscillator of a toner density controller easily when toner density is measured by using a detection coil, by composing the oscillator of an amplifying element with phase characteristics and varying a source voltage.

CONSTITUTION: A power source circuit 27 is equipped with a three-terminal regulator 28 and a variable resistor 29 and its output voltage VDD is variable. The oscillator 30 consists of inverting and amplifying elements 31 and 32 composed of C-MOS inverters and having phase characteristics to the source voltage VDD, and the resonance circuit 35 consisting of the detection coil 18 and a resonance capacitor 25. When the toner density varies, the inductance of the coil 18 varies, so the oscillation frequency varies. This is counted by a counter to adjust the supply amount of toner. Variance in oscillation frequency based upon the coil 18 and capacitor 25 is corrected by a resistance 29. The circuit 27 uses an element with temperature dependency to realize temperature compensation as well.