The present invention pertains to a gas sensor comprising a housing (11) in which a sensing element (20) is secured with a part located on the side of the gas to be measured. Said part comes out of the housing (11) and is surrounded, with a spacing, by at least one protective tube provided with a gas opening (30). The protective tube (30) has an outer cylinder housing (31) and at least an inner cylinder housing (32), mounted at a distance from each other and each presenting gas inlets and/or outlets in a space intended for measuring gas. Internal gas openings (39) are provided in the cover surface of the outer cylinder-housing half (32) turned in the direction opposite that of the gas flow. The inner cylinder-housing half (39) provided with internal gas openings (39) faces the outer cylinder-housing half (31) which has no gas opening.