PURPOSE: To build up side and transverse plate members of a case body in the relation of a correct right angle despite a slight amount of difference for mounting the transverse plate member or the side plate further in no relation to a state of application of pressing force to the side plate, by placing a correction jig between the left and right side plates vertically between the transverse plate members or mounting adjusters to lower ends of the side plates and inserting said adjusters to insertion holes provided in a surface plate.

CONSTITUTION: Transverse plate members 2, 2 are arranged between the top and bottom of a pair of side plates 1, 1 respectively placed on a surface plate 4, and a correction jig 3 with its four corners in a right angle is placed between the pair of side plates 1, 1 in a location between said upper and lower transverse plate members 2, 2, then one of the side plates 1 is adapted to the before said surface plate 4 and a vertical retaining board 5, while an external face of the other side plate is pressed to a side of the vertical retaining board, or instead of using the above described correction jig 3, there are provided adjusters 6 at lower ends of the side plates 1, 1 and insertion holes 7 for inserting said adjusters 6 to the surface plate 4.