A gas sensor having a simple structure and capable of accurately measuring the combustible gas component concentration of a measurement gas such as an exhaust gas despite variations in the oxygen concentration of the exhaust gas and variations in element temperature. The gas sensor 1 has the following constituent features: (1) first processing space 9: a measurement gas is introduced thereinto via a first gas passage 11; (2) second processing space 10: a gas contained in the first processing space 9 is introduced thereinto via a second gas passage 13; (3) oxygen concentration detection element 4: adapted to measure the oxygen concentration of a gas contained in the first processing space 9; (4) oxygen pumping element 3: adapted to reduce the oxygen concentration of the measurement gas introduced into the first processing space 9 within a range such that water vapor contained in the measurement gas is not substantially decomposed; (5) oxidation catalyst section 16: adapted to accelerate combustion of a combustible gas component contained in a gas introduced into the second processing space 10; and (6) combustible gas component concentration detection element 5: a constant voltage is applied thereto, and element 5 has an output current which varies according to the amount of oxygen consumed by combustion of a combustible gas component contained in the gas introduced into the second processing space 10 from the first processing space, to thereby provide information regarding the concentration of the combustible gas component of the measurement gas based on the output current.