To secure consistently a required connection area of a filler pipe in a breather pipe by preventing adhesion of a ring-shaped wax material to areas other than predetermined installation surfaces of a breather pipe connection part in welding the breather pipe with the filler pipe by melting and solidifying the ring-shaped wax material, in order to enhance the connection strength and air tightness.

In a breather pipe connection part 4b formed by enlarging the diameter of the end part 4a of the filler pipe 4 on the side of a fuel injection port, an insertion hole 4c into which one end of the breather pipe 5 is inserted is formed, and a recessed part 4d is formed in the breather pipe connection part 4b in the periphery of the insertion hole 4c. The recessed part has a flat bottom wall part 4e surrounding the insertion hole 4c. In a state that the ring-shaped wax material 6 engaged in the breather pipe 5 is put on the bottom wall part 4e, the breather pipe 5 is welded with the filler pipe 4 by melting the ring-shaped wax material 6 so as to make connection opening.