A protrusion (54) acting as an engagement portion is provided to a root (52) of a spindle (5) to an engine. An extending portion (45) extending above the root (52) of the spindle (5) is provided to a stopper (4). An opening (46) acting as a coupler is provided to the extending portion (45). The protrusion (54) and the opening (46) engage with each other, thereby reducing the risk of the spindle (5) coming off and regulating relative displacement of an upper rigid member (2) supporting the spindle (5) inserted into the upper rigid member (2).