an oxygen concentration sensor having an oxygen ion conductive solid electrolyte member forming a gas diffusion restricted region into which a measuring gas is introduced , includes a pump current source for supplying a pump current across a pair of electrodes sandwiching the solid electrolyte through a current detection element and a heater for heating the solid electrolyte . by an arrangement in which the current detection element is connected to one of the pair of electrodes facing the gas diffusion restricted region , error of measurement due to leakage of a drive current of the heater is prevented .