Provided is a gas sensor which preferably suppresses an occurrence of signal abnormality at a start of driving. The gas sensor includes a first space including a reference gas inside, a reference electrode being in contact with the reference gas, and a measurement electrode being in contact with a measurement gas included in a second space separated from the first space, wherein the gas sensor determines a concentration of a predetermine gas component in the measurement gas using a potential difference between the reference electrode and the measurement electrode and includes a metal member formed of a ferritic stainless steel including chrome of 11.00 to 18.00 wt% but not including molybdenum, titanium, and niobium, being disposed at a position so as to face the first space and be able to receive a heat conduction from at least the measurement gas, and a surface roughness of a surface of the metal member facing the first space is 1000 nm or less.