A fastener may include a plate-shaped connecting base, at least one pair of projecting portions respectively formed in the connecting base, and undercut portions formed in the at least one pair of projecting portions. The at least one pair of projecting portions oppositely project in a lateral direction of the connecting base. The at least one pair of projecting portions respectively have first surfaces sloped toward distal ends thereof. The undercut portions are defined by the first surfaces of the at least one pair of projecting portions.