Patent claim: The gas sensor according to Claim 1 wherein the ratio A2A1 is higher than or equal to 1.5 and lower than or equal to 1.9.

Related passage: A gas concentration detection sensor 400 shown in Fig.8 may be used instead of the gas concentration detection sensor 100 according to the abovedescribed embodiment.The gas concentration detection sensor 400 includes a protective cover 420 having a triplelayered structure namely including a first inner protection cover 430 covering over the free end of the sensor element 110 a second inner protection cover 440 covering over the first inner protection cover 430 and an outer protection cover 450 covering over the second inner protection cover 440.The first inner protection cover 430 has an outer circumferential face that is in contact with a metal main clamp 402 and includes a cylindrical largediameter section 432 and a bottomed cylindrical edge section 434.Twelve gas passing apertures 434a are arranged around a side face of the edge section 434.A gas passing aperture 434b is disposed at the center of a bottom face of the edge section 434.The gas passing apertures 434a are arranged at equal intervals such that they are axially vertically in Fig.8 staggered in two levels.The second inner protection cover 440 has an inner circumferential face that is in contact with the metal main clamp 402 andincludes a cylindrical largediameter section 442 a cylindrical stem section 444 and a bottomed cylindrical edge section 446.The largediameter section 442 is connected to the stem section 444 through a step element 443.The stem section 444 is also connected to the edge section 446 through a step.Six inner gas apertures 443a are arranged at equal intervals on the step element 443.A gas passing aperture 446a is disposed at the center of a bottom face of the section 446.The outer protection cover 450 has an inner circumferential face that is in contact with the metal main clamp 402 and includes a cylindrical largediameter section 452 and a bottomed cylindrical edge section 454 In a boundary portion between a side face and a bottom face of the edge section 454 six circular outer gas apertures 454a are arranged.Each outer gas aperture 454a is formed such that an angle 1 formed by the outer opening plane of the outer gas aperture 454a and the bottom face of the edge section 454 is in a range of 10 degrees to 80 degrees.The edge section 434 of the first inner protection cover 430 is in contact with the edge section 446 of the second inner protection cover 440 such that a space defined by the first inner protection cover 430 and the second inner protection cover 440 is divided into an upper chamber 422 and a lower chamber 424.The gas concentration detection sensor 400 having such a structure offers the following advantages.The structure facilitates the discharge of water entering the outer protection cover 450 to the outside through the outer gas apertures 454a thus preventingfrom entering the second inner protection cover 440 and the first inner protection cover 430 and adhering to the sensor element 110.The outer gas apertures 454a the object gas entering the outer protection cover 450 through the outer gas apertures 454a to flow toward the base end of the sensor element 110 in Fig.8.Since the inner gas apertures 443a are located closer to the base end of the sensor element 110 than the outer gas apertures 454a as illustrated in the figure the object gas entering through the outer gas apertures 454a tends to reach the inner gas apertures 443a.The structure reduces the time to replace a gas in the protection cover 420 with the object gas and thereby enhances the response of the gas concentration detection sensor.