DEVELOPING DEVICE

PURPOSE:To prevent magnetic material carriers from staying nearby a detection surface and to accurately detect the toner concentration of a two-component developer by arranging the projection surfaces of magnetic poles of magnets for conveyance to a rotary sleeve and the projection surface of the concentration sensor on the detection surface so that they do not overlap each other. CONSTITUTION:The projection surfaces of the magnets for conveyance on the rotary sleeve 18 and the projection surface of their detection surfaces on the rotary sleeve 18 are arranged outside the rotary sleeve and between a blade 26 and the magnets for conveyance so that they do not overlap each other; and the density sensor 28 is provided which detects the concentration of the two-component developer from variation in inductance. Therefore, when a magnetic brush passes right above the concentration sensor 28 through the rotation of the rotary sleeve 18, the variation in the inductance of the coil 32 of the concentration sensor 28 is detected to detect the toner concentration of the formed magnetic brush. The variation in the inductance of the coil 32 is proportional to the rate of magnetic carriers in the two-component developer, i.e. toner concentration. Consequently, the accurate toner concentration of the developer can be detected.