A gas sensor, comprising: a first space being an inner space for inclusion of a reference gas including air; a sensor element including: a gas distribution part to which a measurement gas is introduced, said gas distribution part being provided on one end of said sensor element; a reference gas inlet space to which said reference gas is introduced, 10 said reference gas inlet space being provided on the other end of said sensor element; a reference electrode for being in contact with said reference gas, said reference electrode being provided at a position to be communicated with said reference gas inlet space; and a measurement electrode for being in contact with said measurement gas included in a second space, which is separated from said first space, said measurement electrode being provided in said gas distribution part; a plurality of annularly-mounted members (7, 8, 9) being annularly mounted to a periphery of said sensor element; a metal member being annularly mounted to a periphery of said annularly-mounted members (7, 8, 9) and being able to receive a heat conduction from at least said measurement gas, said metal member facing and contacting said first space; a cylindrical cover which is a cylindrical member partitioning said first space and the outside of the gas sensor, said first space being formed inside said cylindrical cover; wherein said gas sensor obtains a concentration of a predetermined gas component in said measurement gas using a potential difference between said reference electrode and said measurement electrode, said metal member is formed of a ferritic stainless steel including chrome of 11.00 to 18.00 wt% but not including molybdenum, titanium, and niobium, and a surface roughness Ra of a surface of said metal member facing said first space is 1000 nm or less. a first space being an inner space for inclusion of a reference gas including air; a sensor element including: a gas distribution part to which a measurement gas is introduced, said gas distribution part being provided on one end of said sensor element; a reference gas inlet space to which said reference gas is introduced, 10 said reference gas inlet space being provided on the other end of said sensor element; a reference electrode for being in contact with said reference gas, said reference electrode being provided at a position to be communicated with said reference gas inlet space; and a measurement electrode for being in contact with said measurement gas included in a second space, which is separated from said first space, said measurement electrode being provided in said gas distribution part; a gas distribution part to which a measurement gas is introduced, said gas distribution part being provided on one end of said sensor element; a reference gas inlet space to which said reference gas is introduced, 10 said reference gas inlet space being provided on the other end of said sensor element; a reference electrode for being in contact with said reference gas, said reference electrode being provided at a position to be communicated with said reference gas inlet space; and a measurement electrode for being in contact with said measurement gas included in a second space, which is separated from said first space, said measurement electrode being provided in said gas distribution part; a plurality of annularly-mounted members (7, 8, 9) being annularly mounted to a periphery of said sensor element; a metal member being annularly mounted to a periphery of said annularly-mounted members (7, 8, 9) and being able to receive a heat conduction from at least said measurement gas, said metal member facing and contacting said first space; a cylindrical cover which is a cylindrical member partitioning said first space and the outside of the gas sensor, said first space being formed inside said cylindrical cover; wherein said gas sensor obtains a concentration of a predetermined gas component in said measurement gas using a potential difference between said reference electrode and said measurement electrode, said metal member is formed of a ferritic stainless steel including chrome of 11.00 to 18.00 wt% but not including molybdenum, titanium, and niobium, and a surface roughness Ra of a surface of said metal member facing said first space is 1000 nm or less.