A gas sensor includes a gas detection chamber into which a detection gas is introduced, a detecting element and a compensating element each disposed in the gas detection chamber, a detection device which detects the concentration of a detection target gas contained in the detection gas, in accordance with a difference in electrical resistance values between the detecting element and the compensating element, and a dehumidifying element which is provided in the gas detection chamber, and reversibly absorbs water contained in the detection gas and drains off the water absorbed in the dehumidifying element.