To provide a valve casing seat structure of a butterfly valve which prevents water leakage or restriction of a valve element attributable to deformation of a valve easing.

A valve casing seat 27 disposed on an inner circumferential surface of a valve casing 12 of the butterfly valve comprises a cylindrical seat ring 28 slidable to a valve element seat 20 of a valve element 15, and an O-ring 30 to seal a space 29 between an outer circumferential surface of the seat ring 28 and the inner circumferential surface of the valve casing 12 on the upstream side and the downstream side of a flow passage 26. The O-ring 30 is formed of a rubber expandable in the radial direction of the seat ring 28, located at a center in the axial direction of the seat ring 28, and faces the valve element seat 20 of the fully closed valve element 15 via the seat ring 28.