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 2 is located at a full-closed position facing a peripheral lower surface 2H of a semicircular portion with respect to a rotating shaft 2B of the butterfly valve 2 made of a resin material, a partial annular projection 1C is adapted to radially inwardly extend towards an inner wall surface 1A of a fluid path 1, the partial annular projection 1C including a plane 1K that contacts the peripheral lower surface 2H of the valve 2 to form a fluid seal. Preferably, while the valve 2 opens only by the thickness thereof from a mechanically full-closed position, from which the valve 2 cannot mechanically rotate any more in a close direction, a surface on the periphery 2E of the valve 2 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 3, 3A, 3B are mounted around the shaft 2B in a bearing hole 1N, and an edge 3H on the fluid path side of the sealing members 3, 3A, 3B elastically contacts an annular surface 2J formed at a position facing the bearing hole 1N about the rotating shaft 2B of the valve 2.