<p>A thermal air flow rate sensor comprising two heating resistors of temperature-sensitive resistor juxtaposed in an air flow along the flow direction thereof while being connected electrically in series, wherein a constant voltage source connected with the two heating resistors supplies a heating current thereto. A control circuit controls heating current supplied to the two heating resistors such that the potential at the joint thereof is equal to a specified referential potential, and an operational amplifier operates the difference between heating currents flowing to the two heating resistors as the air flow rate. A bridge circuit is formed of the two heating resistors and two other resistors, and the referential potential is generated by the power supply means and the two other resistors.</p>
