A detecting device for detecting the amount of toner in dual component developing material is less than the predetermined level in a developing device for an electrostatic copying apparatus which comprises a hopper (4) for supplying the toner (5), a stirring roller (9) for stirring a carrier and the toner (5) from the hopper (4), detecting means for detecting the amount of developing material provided near the stirring roller (9) to mix the toner (5) supplied from the hopper (4) with the carrier, discriminating means for discriminating the output signal from the detecting means for predetermined interval during the rotational period of the stirring roller (9), and an indicating element (46) in response to the output signal from the discriminating means for indicating the amount of the toner (5) in developing material is less than the predetermined level. Since it is indicated when the toner (5) in the developing material is more reduced than the predetermined level, the toner (5) is supplied according to the indication, therefore, the undesirable detrioration of the copy density is prevented.