Arranged are a selective reduction catalyst for reacting NOx in exhaust gas 2 with ammonia, an exhaust passage 5 for guiding the exhaust gas 2 to the selective reduction catalyst, and a urea water addition unit 4 incorporated in the exhaust passage 5 for adding urea water 3 into the exhaust gas 2. A portion of the exhaust passage 5 adjacent to an added position of the urea water 3 is constituted by a divisional piece 5a made of material having higher corrosive resistance to a substance produced by addition of the urea water 3 than that of a material constituting the remaining portion of the exhaust passage 5 other than the adjacent portion.