Abstract: A butterfly valve comprises a substantially cylindrical casing (20) having an annular valve seat (A) on its inner peripheral surface, and a valve disk (44) disposed within the casing (20) and rotatable about an axis (35). The valve disk (44) is formed at an outer peripheral edge portion with an annular groove (52) in which an annular valve seal (51) is partially fitted. The valve seal (51) is adapted for pressing contact with the valve seat (A) at a predetermined pressure and anchored in the annular groove (52) by resin (58) pressure-injected into the annular groove (52) through a resin injection channel (53) in communication with the groove. Accordingly, the seal pressure of the valve seal (51) on the valve seat (A) can be set to the predetermined value at all times even if the dimensions of the valve seat (A) and the valve disk (44) and the position of the axis of rotation (35) involve errors.