An elastically deformable bridging piece includes an engaging protruding part between two ends of the piece, insertion of a portion of an attaching member in a direction intersecting a longitudinal direction of the bridging piece is permitted by elastically deforming the bridging piece by contacting with the engaging protruding part with the portion, and the engaging protruding part engages an engaging hole formed in the portion at an insertion terminating position by elastically returning the bridging piece. A restricting part restricting elastic deformation of the bridging piece is provided. When inserting the portion, the restricting part inclines the bridging piece to position the engaging protruding part on the obliquely downward side, and when a force in a reverse direction to the inserting direction acts on the portion located at the insertion terminating position, the restricting part restricts inclination of the bridging piece, which is reversely-oriented to the former inclination.