Provided is a gas sensor capable of accurately obtaining a hydrocarbon gas concentration even if a measurement gas contains water vapor. A main pumping cell adjusts an oxygen partial pressure of a first internal space such that a hydrocarbon gas in a measurement gas is not substantially burned in the first internal space. An auxiliary pumping cell adjusts an oxygen partial pressure of a second internal space such that inflammable gas components except for hydrocarbon are selectively burned. Then, a measuring pumping cell supplies oxygen to a surface of a measuring electrode. As a result, hydrocarbon existing in the measurement gas being in contact with the measuring electrode is all burned on the surface of the measuring electrode and, based on the magnitude of a current flowing between the measuring electrode and a main pumping electrode at that time, identifies a concentration of a hydrocarbon gas component existing in the measurement gas.