A fuel tank cover (150) is provided with a flange unit (22) closing an opening of a fuel tank, and a joint member (24) configured to be connected to the flange unit (22) movably in the upward/downward direction. The flange unit (22) includes a side tubular portion (43) extending in the upward/downward direction. The joint member (24) includes a columnar portion (160) extending in the upward/downward direction. The columnar portion (160) is inserted into the side tubular portion (43) movably in the upward/downward direction. A metal retainer (162) is provided to cover an outer circumferential surface of the columnar portion (160).