PURPOSE: To improve the precision of detection by detecting a phase difference between a voltage and a supply voltage based on the change in reactance when the change in reactance corresponding to the quantity of toner is detected.

CONSTITUTION: A bridge circuit is formed of a capacitance C of a detection electrode changing in accordance with the quantity of toner and of resistors 3, 4 and 5. By forming a detection circuit in this way, the phase of a deviation voltage VD at intermediate points P and Q is also changed in accordance with the change in the capacitance C. Then, a reference value of the capacitance C is set so that the phase of the deviation voltage VD at the intermediate points P and Q is delayed by α from the phase of a voltage VO of an AC supply source 1. The phase difference at this time being taken as a reference, a change in phase difference from the reference phase difference α is detected. Since the change in reactance is converted into the change in phase difference in this way, the precision of detection can be improved.