Controlling device for developer concentration

PURPOSE:To maintain concentration of developer uniform and enable good developing at all times by detecting the concentration of developer in a developing means, and performing control to replenish deficient developer. CONSTITUTION:A developing device 1 is provided with a fixed magnet 2 and a nonmagnetic sleeve 3 installed concentrically and rotatably in its outer periphery. Developer T held on the peripheral face of the sleeve 3 is given to a photosensitive drum 4 with rotation of the sleeve 3, and an electrostatic latent image formed on the peripheral face of the photosensitive drum 4 is changed to a visible image. Concentration of the developer is detected as reflective concentration by a reflective concentration detector 5 provided near the peripheral face of the sleeve 3. Output of the detector 5 is led to a comparison controlling circuit 8 and drives a motor 9 by signals corresponding to the concentration of the developer. A screw conveyor 11 is then rotated through a gear mechanism 10, and toner T1 in the supplying hopper 12 is supplied to the developing section.