A measuring electrode and a reference plate are formed on the front and rear surfaces, respectively, of the electrolytic plate. The vent plate is provided with a vent channel. The heater base comprises a heating element. The electrolytic plate is made of a green sheet of zirconium oxide consisting of 5 to 7 molar % Y.sub.2 O.sub.3 and 0 to 5 parts by weight of aluminum oxide. The average particle diameter of the zirconium oxide powder is less than 2.0 .mu.m. The green sheet of zirconium oxide has a thickness of 50 to 300 .mu.m. The heater base is made of a green sheet of aluminum oxide consisting of aluminum oxide powder having an average particle diameter of less than 1.0 .mu.m and 0 to 10 parts by weight of zirconium oxide or yttria-stabilized zirconium oxide. The green sheet of aluminum oxide is at least 4 times the thickness of the green sheet of zirconium oxide. To complete the oxygen sensor, these four components are sintered together at 1300.degree. to 1600.degree. C. to be integrated.