Abstract: A gas sensor includes a sensor element, an inner protective cover having inside a sensor element chamber and having an element chamber inlet and an element chamber outlet, and an outer protective cover having an outer inlet and an outer outlet. A total cross-sectional area A [mm2] of the outer inlet, a total cross-sectional area B [mm2] of the element chamber inlet, a total cross-sectional area C [mm2] of the element chamber outlet, and a total cross-sectional area D [mm2] of the outer outlet satisfy B>A>C>D, a cross-sectional area ratio A/D is greater than a value of 2.0 and less than or equal to a value of 5.0, and A×B×C×D is greater than or equal to a value of 3000 and less than or equal to a value of 8500.