To prevent a bellows pipe shaped cover body from deforming while avoiding extreme increase of a weight and raise of costs by integratedly forming a wall thickness reinforcing part which is integratedly formed in a bellows pipe shape with an elastic resin material, and for suppressing folding deformation inward a pipe radial direction of a pipe circumferential wall part, on an annular crest part of the pipe circumferential wall part.

Right and left engaging parts 25a engaged with boss parts 22a, 5a, 18a are formed on right and left both end parts in a pipe longitudinal direction. In between both engaging parts 25a, annular crest parts 25b and annular trough parts 25c having a diameter smaller than that of the crest part 25b are mutually positioned in front and rear parts of the crest part 25b, and a surface of the pipe circumferential wall part is formed in a wave shape. In a crest part (b) of the pipe circumferential wall part, wall thickness reinforcing parts 25d for suppressing recessed deformation caused by folding inward in the pipe radial direction of the pipe circumferential wall part are formed along a circumference of the crest part (b). In the reinforcing parts 25d, each cross section is formed in a nearly square shape, and is formed integrally by the same material with the pipe circumferential wall of the bellows pipe shaped cover body 25.