To provide a novel means for suppressing water-induced cracking of a sensor element of an exhaust gas sensor and other damages caused by a temperature difference.

A droplet diameter reducing part provided for reducing diameters of droplets in exhaust gas is arranged in an exhaust passage in an upstream side of a fixing point of the exhaust gas sensor. A mesh screen 20 being one example of the droplet diameter reducing part is a disc comprised of a heat resistant and anticorrosive mesh body such as SUS, and it has gas permeability. The mesh screen 20 can be turned by an actuator 22. Even if water is formed in the exhaust passage, the droplets D are blown and moved by an exhaust gas flow G, colliding with the mesh screen 20, and they are atomized by the impact.