Abstract: In the valve casing according to the invention, a cylindrical elastic member is fitted to solidly coat a whole inner surface of the valve casing to form a passage, while performing a function as a seal ring for pipe lines and a function as a seat ring for a valve disc.

Abstract: This invention is intended to solve the problem that a valve casing for use in a butterfly valve formed by press working, which is advantageous from the standpoint of productivity, thickness and weight, has such as a serious disadvantage of breakage or deformation and lowering in the sealing performance due to release on aging or the seal ring when the valve is mounted on pipe line and clamped from both sides. From this point of view, a valve casing according to this invention is formed into a double structure composed of an inner shell and an outer shell, the inner shell comprising a cylindrical part defining a bore and seal ring receiving portions formed on both ends with their diameters enlarged larger than the cylindrical part substantially forming an S-shape in cross section, the outer shell defining a space by surrounding a whole outer periphery of the inner shell, and the inner shell and the outer shell being superposed to each other at both end of the valve casing.