When mounting a second assembly 36 to a first assembly 35 that is fixed to a main body, the second assembly 36 engages a second connection element 54 of a non-rotation unit 361 of the second assembly 36 with a first connection element 354 of the first assembly 35. A rotating part 362 is mounted on the first assembly 35 when the rotation unit 362 is rotated. Therefore, an operator can carry out positioning for the second assembly 36 by connecting the second connection element 54 of the second assembly 36 with the first connection element 354 of the first assembly 35. The second assembly 36 can be smoothly mounted on the first assembly 35 that is fixed to the main body. The second connection element 54 of the non-rotation unit 361 connects with the first connection element 354 of the first assembly 35, so that the non-rotation unit 361 is prevented from rotating together with the rotation unit 362 during use and the assemblies that contact the non-rotation unit 361 are protected from damage.