Butterfly valve device

An object of the present invention is to provide a butterfly valve device made of resin, the butterfly valve device being little susceptible the secular change in gas flow (in particular, the minimum flow) or random changes. According to the present invention, when a butterfly valve is located at a full-closed position facing a peripheral lower surface of a semicircular portion with respect to a rotating shaft of the butterfly valve made of a resin material, a partial annular projection is adapted to radially inwardly extend towards an inner wall surface of a fluid path, the partial annular projection including a plane that contacts the peripheral lower surface of the valve to form a fluid seal. Preferably, while the valve opens only by the thickness thereof from a mechanically full-closed position, from which the valve cannot mechanically rotate any more in a close direction, a surface on the periphery of the valve is formed in a curved surface shape having the specific curvature so that the specified minimum airflow is achieved at the mechanically full-closed position. In addition, cylindrical elastic sealing members are mounted around the shaft in a bearing hole, and an edge on the fluid path side of the sealing members elastically contacts an annular surface formed at a position facing the bearing hole about the rotating shaft of the valve.