To normally tighten a bolt and a nut to couple a movable flange to a fixed flange in a tilted state.

The fixed flange 10 is formed around a receiving port of a valve box 4, the movable flange 3 is disposed to face the fixed flange 10, an insertion port 6 is formed in the movable flange 3 to be inserted into the receiving port 5, and the inserting port 6 is engaged in the receiving port 5 with clearance. A rubber ring 7 is disposed between an inner circumferential surface of the receiving port and an outer circumferential surface of the inserting port, and an adjusting bolt 21 disposed between the fixed flange 10 and the movable flange 3 facing each other is inserted into a bolt hole 22 in each flange to be tiltable to an axial center. An adjusting washer 25 is provided between an adjusting nut 24 installed on the adjusting bolt 21 and the movable flange 3. Adjoining surfaces of the adjusting nut 24 and the adjusting washer 25 to each other are formed as spherical surfaces 24a and 25a.