In an oxygen sensor which is provided with an oxygen pump element having electrodes on both front and rear surfaces of a solid electrolyte plate with oxygen ion conductive property, voltage from an external power source is applied between the electrodes and a pumping current flows between the electrodes. Thereby oxygen is pumped out of ambient gases to be measured through a member including a gap with oxygen diffusion limiting property. The limit value of the pumping current accompanied by the oxygen pumping is determined or the oxygen concentration difference produced before and after the diffusive motion of oxygen through the member or oxygen concentration produced after the diffusive motion of oxygen through the member is measured. Thereby the oxygen concentration of the ambient gases is calculated. The oxygen sensor comprises a heater unit installed on the solid electrolyte plate of the oxygen pump element through an electric insulation layer, and a lead wire unit of one electrode of the oxygen pump element connected at one end to the heater unit. Thus the oxygen sensor is excellent in energy saving, detection accuracy, and reduction of use amount of electrode lead wires.