To enhance installation strength of a bush, by restraining slipping-out of the bush from a boss member.

An inside tight fit part 15C is arranged in an inner cylinder part 15A of the bush 15. A part between a bush fitting hole 14A of a stepped boss cylindrical body 14 and the inside tight fit part 15C, and a part between a small diameter cylinder part 14C of a boss cylindrical body 14 and an outside tight fit part 15E, are tightly fitted over two places, by arranging the outside tight fit part 15E in an outer cylinder part 15D of the bush 15. Thus, restraint of the respective bushes 15 to the boss cylindrical body 14 is increased, and the boss cylindrical body 14 and the respective bushes 15 can be firmly integrated. As a result, when a large load repeatedly acts on an end part of the bushes 15, since the bushes 15 and the boss cylindrical body 14 are integrally deformed, only the bushes 15 are not largely deformed to the boss cylindrical body 14, and the slipping-out of the bushes 15 from the boss cylindrical body 14 can be surely restrained.