PROBLEM TO BE SOLVED: To implement retightening with a simple constitution while suppressing increase of costs.SOLUTION: In a double eccentric butterfly valve 10, a center line of a valve rod which is a center of rotation of a valve element 14, is eccentric from a center line of a valve chest valve seat 21 and eccentric from a center line of a flow channel 12 of a valve chest 13. A valve seat 24 of the valve element 14 has a contact portion 25 at a position eccentric to a diameter-expanding side of a tapered face 21a of the valve chest valve seat 21 with respect to a center line O in a thickness direction of the valve seat. The contact portion 25 is formed into the tapered shape by cutting both ends in the thickness direction in the tapered shape in a sectional view. The tapered face 21a of the valve chest valve seat 21 is formed to be expanded to an upstream side of the flow channel 12 of the valve chest 13. As the valve element 14 is closed in a state in which the contact portion 25 of the valve seat 24 bites into the tapered face 21a of the valve chest valve seat 21, it can be retightened.
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