Assembly of a pin guide member for retaining a pair of connecting pins is facilitated for improvement in an assembling property and cost reduction. On the premise that inner brackets are overlapped with outer brackets, and pins are inserted into pin holes thereof so as to connect the brackets to each other, inner collar portions of pin guide members for retaining the pins are combined and integrated with each other by bolts and nuts through a supporting plate serving as a connection member. Meanwhile, outer collar portions of both the pin guide members are supported in the pin radial direction with respect to the inner brackets and rotation thereof is prevented.