The tip face 411 of the base portion 41 inserted into the retaining groove 321 and the groove bottom face 322 have a positional relationship of facing each other, and a bottom portion space 413 is formed between the base portion 41 inserted into the retaining groove 321 and the groove bottom face 322. A pair of side faces 412 of the base portion 41 inserted into the retaining groove 321 and the pair of groove side faces 323 have a positional relationship of facing each other, and side face spaces 324 which are in communication with an opening of the retaining groove 321 are respectively formed between the pair of side faces 412 of the base portion 41 inserted into the retaining groove and the pair of groove side faces 323, and the bottom portion space is in communication with the side face space.