The present invention provides 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. 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 peripherallower 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.