The frame 6 includes a top frame 6a, a reverse frame 6b, a right frame 6c, a left frame 6d, a front frame 6e, and a rear frame 6f which are separate components. The left frame 6d is moved in the direction of an arrow C to the front frame 6e and the rear frame 6f. Ribs provided at both end portions of the left frame 6d are inserted into ribs provided at end portions of the front frame 6e and the rear frame 6f. Similarly, the right frame 6c is also moved, and ribs are inserted into each other. In this manner, the four side frames are assembled. Then, the top frame 6a and the reverse frame 6b are assembled thereto. Since the ribs are inserted into each other as described above, the frame 6 is easily assembled and has excellent robustness.