A toner density detecting device has NESA glass, a bias voltage source, a resistor and/or a varistor or Zener diode, and a luminosity detecting circuit which detects the density of toner adhering to the NESA glass using a light source and a photodiode as a light receiving element. The resistor permits detection at a high voltage without causing breakdown while the provision of the varistor in parallel with the resistor permits the application of constant voltage to the NESA glass. In addition a toner supply device has a detector and a toner supply circuit which compares the detection signal from the detector with two reference signals included therein to supply either a constant amount of toner or supply the toner for a predetermined period of time.