To manufacture a fiber-reinforced synthetic resin molding like a pipe member or a U-shaped gutter having no restriction in its length by satisfying mechanical strength required for its real use even if a steel pipe is not used as a core material.

A coating layer 6 of synthetic resin is formed by heat welding on an inner or outer periphery of a core material 5 having a hollow cylindrical or open sectional shape formed of fiber-reinforced synthetic resin obtained by mixing reinforcing fiber such as glass fiber with synthetic resin such as polypropylene resin. The material 5 is obtained by bending a sheet of fiber- reinforced synthetic resin and forming a pipe or bending it and forming it in an open sectional shape.