PURPOSE: To reduce remarkably the occurrence of wrinkles or imbalance of the configuration of lug sections, by separating a reinforcing core for a gutter into a core material for reinforcing a barrel section and core materials for reinforcing lug sections, and associating them before a step of covering with a synthetic resin.

CONSTITUTION: A belt-shaped core material 10 is formed by a molder A into the shape of a gutter, that is, a core material 1 for reinforcing a barrel section, the core material 1 having opposite side sections 11, 11. Rodlike core materials 2 having longitudinal recesses 21 for reinforcing lug sections are fitted onto the side sections 11, 11 of the core material 1. Then all the circumferences of the core materials 1, 2 in a synthetic resin extruder are covered with an extruding synthetic resin material.