Abstract: A gas concentration detecting device for detecting the concentration of a component, such as an oxygen in an exhaust gas of an internal combustion engine. The device has a concentration detecting body 2 for detecting the oxygen and a cover 3 of a double wall structure which is for encircling the portion of the detecting body opened to the flow of the exhaust gas to be detected. The cover 3 has, at its axial tip end wall, a plurality of holes 33 and 34 arranged in such a manner that one of the holes functions as a gas inlet hole and the other hole functions as a gas outlet hole in order to cause the gas to be smoothly ventilated. The cover has, at its side, a wall of a closed structure with no hole for the passage of the gas, which is effective for preventing the cover 3 and the detecting body 2 from being cooled by the exhaust gas, thereby enhancing a heat retaining capacity, thereby preventing particulates from being deposited.