PURPOSE: To reduce the cost of the transport of products, while its production equipment may be simplified and the production facility can be located near the consumer's districts by a method in which the bandlike mat produced by mixing the fiber of thermoplastic resin with reinforcing fiber is formed, and this mat is wound on a mandrel in preheated state, and then while the mat is pressurized by a holding roll from outside, the mat is wound up.

CONSTITUTION: The special fiber materials such as cellulose fiber, glass fiber and polyolefin resin are mutually mixed at a prescribed ratios, and the mixture is manufactured into a sheet shape by a wet making process. The beltlike mat 1 wound up into a roll shape passes through between plurality of pairs of rolls 21 of preheating part 2, and is preheated at about 150°C. The mat is wound in partially lapped condition around a rotating mandrel 3 from reward obliquely in the molten state of resin component. The mat is wound up, while the mat is pressurized and cooled by the holding roll 4 rotating reversely to the mandrel from the outside of the wound mat 1. By the resin in molten state, the mat is formed into an integral spiral pipe A, and is separated from the mandrel when it is cooled to at most 80°C.