PURPOSE: To stably control a toner mixing ratio and to improve printing quality by suppressing the fluctuation in the output of the detected toner mixing ratio of the system for detection of the mixing ratio of a developing device having a developer agitating member having agitating vanes for agitating and mixing the developer consisting of a toner and a carrier and a toner mixing ratio detecting means for detecting the magnetic permeability of the developer and determining the toner mixing ratio therefrom.

CONSTITUTION: This system is constituted by having the developer agitating means 15 consisting of a rotating body having the agitating vanes 16 made of a nonmagnetic material for agitating and mixing the developer 4 and the toner mixing ratio detecting means 59 which is provided near this developer agitating member 15, detects the toner mixing ratio of the developer 4 as a magnetic permeability, converts this permeability to a voltage and has a smoothing circuit 56 set with the time constant at the pass period when the above-mentioned agitating vanes 16 pass or above in the output part thereof.