Patent claim: The gas sensor according to any one of Claims 1 to 5 wherein the inner protective cover includes a guide portion that regulates a flow of a gas to be measured flowing into the inner protective cover through the first inner gas hole and wherein a ratio R2R1 defined by a ratio of a radius R1 representing a radius of a circumscribed circle of the sensor element where the circumscribed circle is coaxial with the inner protective cover and a radius R2 representing a radius of an inscribed circle of a plane including a regulation surface of the guide portion that regulates the flow of the gas to be measured where the inscribed circle is coaxial with the inner protective cover is higher than or equal to 1 and lower than or equal to 2.38.

Related passage: The outer protection cover 140 is a metal for example stainless steel member having a cylindrical largediameter section 142 a cylindrical stem section 144 connecting to the largediameter section 142 and having a smaller diameter than the diameter of the largediameter section 142 and a bottomed cylindrical edge section 146 having a smaller diameter than the diameter of the stem section 144.The outer protection cover 140 further has a step element 145 connecting the stem section 144 with the edge section 146.The largediameter section 142 thestem section 144 and the edge section 146 each have a central axis identical to that of the inner protection cover 130.The largediameter section 142 has an inner circumferential face that is in contact with the main clamp 102 and with the largediameter section 132 such that the outer protection cover 140 is fastened to the main clamp 102.The stem section 144 is located to cover over an outer circumferential face of the first stem section 134 and that of the second stem section 136.Six first outer gas apertures 144a formed to communicate with the outside of the outer protection cover 140 and with the first gas chamber 122 are arranged at equal intervals around the stem section 144.The first outer gas apertures 144a are circular holes located in a first corner 144b serving as a boundary portion between a side face of the stem section 144 and a bottom face of the step element 145.The first outer gas apertures 144a are formed such that the outer opening plane of each first outer gas aperture 144a forms an angle of 45 degrees with the bottom face of the 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.Although the first outer gas apertures 144a are formed such that the outer opening plane forms an angle of 90 degrees with the inner circumferential face the embodiment is not especially limited to the arrangement.For example when an angle of 90 degrees or more is formed advantages of the present invention are achieved.Fig.4 is an enlarged partial sectional view of a portion including the first outer gas aperture 144a inFig.2.As shown in Fig.4 the outer opening plane broken line a of the first outer gas aperture 144a forms an angle of 45 degrees with the bottom face broken line b of the step element 145.The inner circumferential face 144c of the first outer gas aperture 144a forms an angle of 90 degrees with the outer opening plane broken line a thereof.The edge section 146 is located to cover over the edge section 138 and an inner circumferential face of the edge section 146 is in contact with an outer circumferential face of the second stem section 136.Six second outer gas apertures 146a formed to communicate with the outside of the outer protection cover 140 and with the second gas chamber 126 are arranged at equal intervals in a second corner 146b serving as a boundary portion between a side face and a bottom face of the edge section 146.The second outer gas apertures 146a are circular holes formed in a manner similar to the first outer gas apertures 144a such that the outer opening plane of each second outer gas aperture 146a forms an angle of 45 degrees the bottom face of the 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.The positional relationship between each of the second outer gas apertures 146a and corresponding one of the gas passing apertures 138a is defined such that the gas passing aperture 138a is located at a position other than the extension of the second outer gas aperture 146a.Fig.5 is an enlarged partial sectional view of a portion including the second outer gas apertures 146aand the gas passing apertures 138a in Fig.2.As shown in Fig.5 when light having directivity is virtually applied from a direction along the central axis of each second outer gas aperture 146a a direction which forms an angle of 45 degrees with the direction along the central axis of the outer protection cover 140 a region 146c irradiated with the light appears on a bottom face of the edge section 138 of the inner protection cover 130.This region 146c is referred to as the region on the extension of the second outer gas aperture 146a.The gas passing aperture 138a is located in a position other than the region 146c.The six first outer gas apertures 144a have the same area of opening and the six second outer gas apertures 146a have the same area of opening.The opening area of each second outer gas aperture 146a is larger than that of the first outer gas aperture.Since the first outer gas apertures 144a are equal in number six to the second outer gas apertures 146a therefore the total area the area of each aperturethe number of apertures of the second outer gas apertures 146a is larger than that of the first outer gas apertures 144a.The first outer gas apertures 144a each has a circular cross section perpendicular to the central axis of the first outer gas aperture 144a and the second outer gas apertures 146a each has a circular cross section perpendicular to the central axis of the second outer gas aperture 146a.The first outer gas apertures 144a and the second outer gas apertures 146a may have a diameter that is not specifically restricted for example the first outer gasapertures 144a have a diameter in a range of 0.8 to 12 mm and the second outer gas apertures 146a have a diameter in a range of 0.8 to 1.2 mm in the present embodiment.