An oxygen sensor particularly useful for detecting oxygen concentration in exhaust gas of internal combustion engine is principally made up of a solid electrolyte tube which is closed at one end, anode and cathode electrode layers respectively formed on the outer and inner surfaces of the electrolyte tube and a tubular metal shell which tightly receives therein the electrolyte tube, leaving a closed end portion of the electrolyte tube protruded from the shell. To protect the protruded portion of the electrolyte tube against a direct exposure to a high velocity hot gas stream to be measured and a collision with any external article, without obstructing a rapid and uniform heating of the electrolyte tube by the gas stream, a hood in the shape of a cylindrical tube with a closed end is fixed at its open end portion to the shell to enclose therein the closed end portion of the electrolyte tube. The hood has a single gas inlet aperture formed in the side wall to extend axially of the hood and open against the gas stream and at least one gas outlet aperture formed in the side wall circumferentially at approximately 180.degree. from the inlet aperture. The total area of the outlet aperture is smaller than the area of the inlet aperture.