Abstract: A toner concentration detecting apparatus comprises a rotatable probe having a toner-receiving face, a case housing the probe, means attached to the case for detecting an amount of toner on the toner-receiving face, and a port formed in the case for discharging stray toner particles from the case. The toner-receiving face is put in contact with a magnetic brush of a developer mechanism, separates toner particles from a developer and receives them. The detecting means includes a light-emitting element for applying light to the toner-receiving face and a light-receiving element for receiving the light reflected by the toner-receiving face and converting the light into an electric signal. The stray toner discharging port extends in the direction in which acts centrifugal force generated by the rotation of the probe and is located near the light-emitting and light-receiving elements.