A collar member formed as a hollow cylinder having an opening at one end into which a mounting pin member of the damper unit is inserted, and which is secured to a cored bar member, a seat part provided at the other end of the collar member in which the insertion side tip of the pin is seated, a positioning groove part formed on the collar member between the seat part and the opening, a locking groove part formed on the pin that is positioned in the positioning groove part when the insertion side tip is seated in the seat part, and a spring member provided on the cored bar member inserted through the positioning groove part that is elastically locked by the locking groove part when the insertion side tip is seated in the seat part and retains the pin.