PURPOSE: To accurately grasp that toner and carrier are uniformly mixed without stopping a mixing device by mixing them until the outputs of toner concentration sensors attached to the wall surface of a mixing tank at a proper distance apart in an up-and-down direction get nearly equal.

CONSTITUTION: The toner concentration sensors 21-23 are attached to the outer peripheral surface of the mixing tank 11 at the proper distance apart in the up-and-down direction and output voltage based on the intensity of the magnetism of developer near the respective attaching positions in the tank 11. Therefore, the output voltage of the respective toner concentration sensors 21-23 gets high when the rate of the carrier being a magnetic substance is high and the output voltage thereof gets low when the rate of the carrier is low and the rate of the toner is high. The outputs of the respective sensors 21-23 are compared by a comparator 30 and an alarm 40 is driven when the outputs of the respective sensors 21-23 get nearly equal. Thus, that the toner and the carrier are uniformly mixed is accurately grasped without stopping the mixing device.