To prevent patterning shape inferiority by compounding a specific amt. of a silane compd., a tin catalyst, an optical sensitizer and a crosslinking aid with a polyolefinic resin which becomes a base material to mold a foamed resin tube, and irradiating the tube with ultraviolet rays at a stage up to extruding a non-foamed resin coating layer to cover the tube.

0.1-5 pts.wt. of a silane compd. and a proper amt. of a polymerization initiator are added to 100 pts.wt. of a polyolefinic resin to form resin pellets. Next, 0.01-2 pts.wt. of a tin catalyst, 0.1-5 pts.wt. of an optical sensitizer and 0.1-5 pts.wt. of a crosslinking aid are added to 100 pts.wt. of the polyolefinic resin to be kneaded therewith and a volatile org. foaming agent is introduced into the molten kneaded mixture to extrude this mixture from the leading end of an extruder 1 to foam the same to continuously mold a foamed resin tube 2. Next, the foamed resin tube 2 is cooled and the surface thereof is irradiated with ultraviolet rays and a non-foamed surface coating film layer is continuously extruded by a coating mold 7 to be applied to the outer surface of the foamed resin tube 2.