A gas sensor comprising a first measuring chamber for introducing a gas to be measured, a second measuring chamber for detection of the gas to be measured, and as pump cells a first pump cell having a pair of pump electrodes, and a second pump cell having a measuring electrode and an auxiliary pump electrode, wherein a porous alumina sintered body having communicating pores of 500-1100 .ANG. in average pore diameter and 6-16% in porosity is formed as an electrode protective layer on the surface of at least the measuring electrode of the second pump cell in such a manner that the alumina sintered body covers the measuring electrode. The gas sensor can be use for long period of time because of the protective layer.