A gas sensor , comprising: a multilayer body formed of a plurality of oxygen ion conductive dense solid electrolyte layers stacked one on top of one another, the multilayer body having therein an object gas flow path through which an object gas is introduced from one end of the multilayer body , and a reference gas introduction space through which a reference gas is introduced from the other end of the multilayer body , a concentration of a specified gas in the object gas being detected with reference to the reference gas; a measurement electrode formed directly on one of the solid electrolyte layers facing the object gas flow path; a reference electrode formed directly on a surface of the one solid electrolyte layer on which the measurement electrode is formed, the surface being a surface on a side opposite a side on which the measurement electrode is formed; a reference gas introduction layer that covers the reference electrode , the reference gas introduction layer being formed of a porous body and formed so as to be exposed to the reference gas introduction space only on the other end side of the multilayer body at a position further away from the one end than the reference electrode is, the reference gas introduction layer extending along the surface of the one solid electrolyte layer on which the reference electrode is formed and protruding into the reference gas introduction space , the reference gas being introduced through the reference gas introduction layer from the reference gas introduction space to the reference electrode ; a detection unit that detects the concentration of the specified gas in the object gas in accordance with a first electromotive force generated between the reference electrode and the measurement electrode ; and a heater formed on the side of the reference electrode opposite a side on which the one solid electrolyte layer is disposed, the one solid electrolyte layer being a layer on which the reference electrode is formed directly. a multilayer body formed of a plurality of oxygen ion conductive dense solid electrolyte layers stacked one on top of one another, the multilayer body having therein an object gas flow path through which an object gas is introduced from one end of the multilayer body , and a reference gas introduction space through which a reference gas is introduced from the other end of the multilayer body , a concentration of a specified gas in the object gas being detected with reference to the reference gas; a measurement electrode formed directly on one of the solid electrolyte layers facing the object gas flow path; a reference electrode formed directly on a surface of the one solid electrolyte layer on which the measurement electrode is formed, the surface being a surface on a side opposite a side on which the measurement electrode is formed; a reference gas introduction layer that covers the reference electrode , the reference gas introduction layer being formed of a porous body and formed so as to be exposed to the reference gas introduction space only on the other end side of the multilayer body at a position further away from the one end than the reference electrode is, the reference gas introduction layer extending along the surface of the one solid electrolyte layer on which the reference electrode is formed and protruding into the reference gas introduction space , the reference gas being introduced through the reference gas introduction layer from the reference gas introduction space to the reference electrode ; a detection unit that detects the concentration of the specified gas in the object gas in accordance with a first electromotive force generated between the reference electrode and the measurement electrode ; and a heater formed on the side of the reference electrode opposite a side on which the one solid electrolyte layer is disposed, the one solid electrolyte layer being a layer on which the reference electrode is formed directly. <EOS>