The connection member 300 is provided with two first protruding portions 351, 352 and two second protruding portions 361, 362 (FIG. 4B). The first protruding portions 351, 352 are commonly disposed on one of the long lateral surfaces 340 of the connection member 300, and are placed oppositely. That is, one of the first protruding portions 351 is disposed on the first end portion 320 of the connection member 300, and the other of the first protruding portions 352 is disposed on the second end portion 330 of the connection member 300. The second protruding portions 361, 362 (FIG. 4B) are commonly disposed on the other of the long lateral surfaces 340 of the connection member 300, and are placed oppositely. That is, one of the second protruding portions 361 is disposed on the first end portion 320 of the connection member 300, and the other of the second protruding portions 362 (FIG. 4B) is disposed on the second end portion 330 of the connection member 300.