POLYOLEFIN RESIN VALVE

PROBLEM TO BE SOLVED: To improve the stability in the shape of a valve casing in a gate valve and to ensure the water-tightness of the valve casing to prevent the influence by bead by thermal fusing the outer extension pieces among extension pieces to one another and butting the inner extension pieces to one another. SOLUTION: A valve casing is made of a polyethylene resin or the like. The valve casing comprises a valve casing body 2a and a closure body 2b. The mating faces 3 comprise the inner and outer double extension pieces 3a, 3b, the outer extension pieces 3b among these extension pieces 3a, 3b are thermally fused to one another, and the end faces of the inner extension pieces 3a are butted to one another. On this occasion, the outer pieces 3b are deformed while fused to one another, but the inner pieces 3a are contacted with one another without deformed. The deformation of the outer pieces 3b is stopped at that time. A space 3c is used as a heat insulation space to the inner extension pieces 3a in the thermal fusion of the outer extension pieces 3b, and also used as a storage space of the bead 5 produced when the outer extension pieces 3b are fused. A width of this space is determined within a range not reducing the thicknesses of both extension pieces 3a, 3b under the consideration of dimensions for the heat insulation and a size of the bead 5.