An inside projection is formed in one piece with a second tubular member and projects in a tubular form toward a third tubular member, and an outside projection is formed in one piece with the third tubular member and projects in a tubular form toward the second tubular member, while an inner wall of the outside projection is engaged with an outer wall of the inside projection. Another inside projection is formed in one piece with the second tubular member and projects in a tubular form toward a first tubular member, and another outside projection is formed in one piece with the first tubular member and projects in a tubular form toward the second tubular member, while an inner wall of the outside projection is engaged with an outer wall of the inside projection.