PURPOSE: To improve the extractability of a mandrel and prevent the contamination of a mold releasing agent or foreign matters into the inside layer by extruding rubber or plastic on a mandrel consisting of a vulcanized rubber rod to then be cross-linked at high temperature and thereafter drawing the mandrel therefrom.

CONSTITUTION: Nylon is extruded to sheathe a vulcanized ethylene propylene diene rubber rod to then become an inside layer tube 2, and a fiber reinforced layer 3 is provided thereon by knitting polyvinyl alcohol fiber, and further the outside layer of stylene butadiene rubber is extruded on the upper layer to sheathe it. Then, these are collectively vulcanized within a vulcanizing device. Thus, the outer diameter of the mandrel is made thin through extension during the period of drawing the mandrel with the result that the mandrel is drawn smoothly. Inside flaws in the rubber or plastic tube inside layer that might be caused at the time of drawing are completely eliminated, and the contamination of a mold releasing agent and foreign matters can be prevented.