PURPOSE: To make the wall thickness of an expanded resin pipe uniform by performing the expansion of the resin pipe gradually while controlling it through a tapered inner surface.

CONSTITUTION: A heat-shrinkable resin pipe 53 is expanded in a radial outer direction along the tapered inner surface of an inner cylinder 52 in a vacuum expansion chamber after passing through a heating bath of a high lubrication heating oil, and the outer diameter of the resin pipe 53 is aligned constantly through the cylindrical inner surface of the inner cylinder 52. Next, the expanded resin pipe 63 is cooled an hardened directly by the cooling fluid of a cooling device 57 immediately after coming out the vacuum expansion chamber, thereby forming a uniform thickness resin pipe.