To provide a butterfly valve, in which cavitation can be effectively suppressed on the orifice side up to an opening of a medium degree, and which is easily manufactured.
A semi-disc-like sub-valve element 5 having a large number of flow holes 16 is arranged on the downstream side on the orifice side of a valve element 4, and when the sub-valve element 5 integrally rotates with the valve element 4, a peripheral edge part of the sub-valve element 5 slidingly contacts with or approach an orifice side concave spherical surface part 11 of an inner surface of a valve casing body 2a, so that fluid is straightened when passing through the flow holes 16 of the sub-valve element 5 and the cavitation on the orifice side is restrained up to the opening of the medium degree, and the butterfly valve can be more easily and inexpensively manufactured than a conventional butterfly valve of using a plate member having a porous convex spherical surface.
Higashio Masahiro
Takayoshi Tagawa