PURPOSE: To prevent a lower-stage block from damage and unfavorable effect and to reduce repair cost, by changing the welding positions of an upper-stage block on the upside of the middle plate, thati is, the ceiling of the down-stage.

CONSTITUTION: A flash promoting mechanism 5, such as a flash box, necessary for an upper stage is fixed in advance to the prescribed position of the upside of a middle plate 3, that is, the ceiling of a lower-stage block 2. Then, an upper- stage block 1 is hung up above the block 2, and placed so as to locate the partitions 4 of the block 1 on the mechanisms 5 and the lower ends of the partitions 4 of the block 1 are welded to the upper parts of the mechanisms 5. As a result, conventional problems of repair cost and guarantee of quality are solved.