To provide a thermal gas flow sensor capable of reducing changes over time in resistance values of a heating resistor and a temperature measuring resistor and improving detection accuracy.

In a detection element 1, protective films 30a, 30b and 31a: insulating films 31b and 30c; the heating resistor 3; the temperature measuring resistor 4; etc. are formed in a flat substrate 32 constituted of a material having high thermal conductivity. The flat substrate 32 is etched from its back surface to form a lower space 35 at a lower part of the insulating film 30c and form a diaphragm 2. Since the membrane stress of the diaphragm 2 varies due to the desorption of H2O from the uppermost-surface protective film 30a when the heating resistor is heated to vary a resistance value of the heating resistor 3 due to the piezoresistance effect, the protective film 30a is made of an oxynitride film (an SiON film) and a polysilicon film of a nonadsorbing material which contains only a small content of H2O. It is thereby possible to reduce contents of H2O and others, the occurrence of cracks, and variations in resistance values of the heating resistor and the temperature measuring resistor.

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