To prevent false detection of oxygen concentration by inhibiting a drop in temperature of a sensor element part due to low-temperature exhaust gas.

Around a sensor element part 13a of an O2 sensor 13, which is mounted through the circumferential wall of an exhaust duct 10 and is exposed to exhaust gas, a hollow cylindrical shield member 30 is arranged to be substantially concentric therewith. The shield member 30 is perforated at its circumferential wall facing the upstream and downstream sides of the exhaust duct 10 with a plurality of upstream side bores 30a and downstream side bores 30b respectively. The opened area of the downstream side bore 30b is set larger than that of the upstream side bore 30a.