PURPOSE: To detect the remaining quantity of a toner in a toner hopper by detecting a toner mixing ratio of a developer in a developing device by a variation in magnetic permeability of the developer.

CONSTITUTION: A magnetic sensor 50 is constituted of a detecting head 51 for contacting a developer in a developing device and detecting the magnetic permeability of the developer, an amplifier 52, an oscillator 53, a phase comparator for executing a comparison in phase difference of a reference phase signal outputted from this oscillator 53 with a detecting signal amplified by the amplifier 52, and a low-pass filter 55. The larger a toner mixing ratio becomes, the smaller an output voltage of a magnetic sensor 50 becomes. As for the output voltage of the magnetic sensor 50 to an optimum toner mixing ratio Tc for obtaining an image of good quality, the output voltage of the time when the toner quantity has become small, and the output voltage of the time when the toner has come not to be carried at all are derived, set as a comparing voltage of a voltage comparator 56, and compared with the present output voltage of the magnetic sensor 50. In this way, the remaining quantity of the toner in the toner hopper can be detected more exactly.