PURPOSE: To improve deformation due to thermal elongation or contraction, peeling between layers, a lateral strength, and to provide excellent long term durability by covering a core material layer formed by dispersing methylmethacrylate resin among many continuous long fibers with vinyl chloride resin.

CONSTITUTION: Powder methylmethacrylate resin 3 is diffused upward above a porous bottom plate 6 by pneumatic pressure and held in a floating state on a fluidizing floor 7. The resin 3 of a floating state is immersed with many long fibers 3 introduced to the floor 7. It is heated by a heating furnace 8 as required, the resin of at least on the surface is melted, and pressed by a presser 9 to form a core material. The thus obtained core material 4 is formed in a winding body 91, again extracted, introduced into a crosshead mold 11 of an extruding machine 10, and the outer surface of the material 4 is covered with vinyl chloride resin to be melted and extruded from the mold 11 under a pressing pressure.