An air flow detecting element is located in the air flow to be measured. This air flow detecting element comprises a heater made of resistant material having a positive temperature coefficient and wound around a first heat resistant frame, a first heat sensitive element wound around the first frame together with the heater and made of resistant material having a positive temperature coefficient and receiving heat from the heater, and a second heat sensitive element wound around a second heat resistant frame and made of resistant material having a positive temperature coefficient, and serving to measure the temperature of the air. A bridge circuit comprises a series circuit consisting of the heater and fixed resistances connected in series with each other, and a series circuit consisting of the first and second heat sensitive elements connected in series with each other. Potential differences at the output terminal of this bridge circuit are compared by an OP amplifier. The base of a transistor, with controls the power source current relative to the bridge circuit, is controlled by the OP amplifier. An output voltage signal which corresponds to the value of the current flowing to the heater is detected and supplied as a terminal voltage of the fixed resistance.