PURPOSE: To easily insert a wire into a frame pipe by forming a wire inserting hole of connecting wire lead-in/out ports in a synthetic resin foaming layer.

CONSTITUTION: First, wire lead-in/out ports 18, 19 are drilled in a side surface part of a pipe 11. Next, a core unit 30 is inserted respectively through this pipe 11 and the wire lead-in/out ports 18, 19. While holding a condition of extending the core unit 30, the inside of this pipe 11 is filled with a foaming unit to form a foaming layer 20. Thereafter, when the core unit 30 is drawn out and removed, a wire inserting hole 21 is formed between the wire lead-in/out ports 18, 19, in the foaming layer 20.