An object of the present invention is to provide a thermal airflow sensor with high detection accuracy. In achieving the above object, this invention provides a thermal flow sensor including: a flow rate detection element that has a diaphragm formed by processing a semiconductor substrate, a heating resistor provided on the diaphragm, and resistance temperature detectors installed upstream and downstream of the heating resistor; and a support member that adhesively holds the flow rate detection element with a sheet adhesive interposed therebetween. The support member includes a communicating hole of which one end has an opening to a cavity provided on the back side of the diaphragm. The sheet adhesive has a ventilating hole formed in an opening area of the communicating hole in the support member.