Abstract: A toner concentration control device comprises a toner supply mechanism for supplying a toner to a developing device, a probe for picking up the toner in the developing agent after development is effected and a control mechanism for controlling the toner supply mechanism. The probe has parallel grooves on the outer peripheral surface thereof which constitute a toner pickup surface. An amount of toner picked up is optically detected and the control mechanism controls the toner supply mechanism based on the result of detection. By so doing, an amount of fresh toner corresponding to an amount of toner spent by development is supplied from the toner supply mechanism to a developing agent in the developing device to maintain constant the toner concentration in the developing agent in the developing device.