An object of the present invention is to provide a thermal airflow sensor that prevents moisture absorption by a silicon oxide film formed closest to a surface (formed to be located on an uppermost portion), and that reduces a measuring error. In order to attain the foregoing object, the thermal airflow sensor according to the present invention applies an ion implantation to a silicon oxide film 4, formed closest to a surface (formed to be located on an uppermost portion), by using an atom or molecule selected from at least any one of silicon, oxygen, and an inert element such as argon or nitrogen, in order to increase a concentration of an atom contained in the silicon oxide film 4 more than that before the ion implantation.