First outer gas apertures 144a are arranged in a first corner 144b such that the outer opening plane of each first outer gas aperture 144a forms an angle of 45 degrees with a bottom face of a step element 145 and the outer opening plane forms an angle of 90 degrees with an inner circumferential face of the first outer gas aperture 144a. Second outer gas apertures are arranged in a second corner 146b such that the outer opening plane of each second outer gas aperture 146a forms an angle of 45 degrees with a bottom face of an edge section 146 and the outer opening plane forms an angle of 90 degrees with an inner circumferential face of the second outer gas aperture 146a. This structure prevents water from adhering to a sensor element 110 and thereby enhances the response of a gas sensor 110.