PURPOSE: To prevent frost damage effectively.

CONSTITUTION: A primer coating layer 12 is installed to the outer surface of a pile main body by way of an adhesive layer 13. The adhesive layer 13 and the coating layer 14 are formed by jointly pushing out adhesive modified polyolefin resin as the adhesive layer and a coating resin material as the coating layer 14 on the outer surface of the primary coating layer 12 of the pile main body 11. The coating resin material which constitutes the coating layer 14 is required to use a numerical value H of 0.01 and below where the value H is available when the critical surface tension is divided with a squared shore D hardness at a temperature of -10°C.