To provide a thermal type air flow detector, capable of restraining reliability of an air flow signal from getting low, using a semiconductor of polycrystalline structure as heating resistor.

This thermal type air flow detector is provided with a heating resistor Rh, comprising polycrystalline silicon and provided inside a suction duct IM, an operational amplifier OPx or the like for controlling a current Ih flowing in the heating resistor Rh to control heating by the heating resistor Rh, and an operational amplifier OPy or the like for amplifying an air flow signal voltage Vs, based on the heating by the heating resistor Rh to output air flow signal. The operational amplifier OPx or the like controls the heating by the heating resistor Rh, to provide temperature characteristic, having a negative inclination with respect to a change of an ambient temperature of the heating resistor Rh, and the operational amplifier OPy or the like amplifies the air flow signal voltage Vs to provide the temperature characteristic, having a positive inclination with respect to the change of the ambient temperature of the heating resistor Rh.