PURPOSE: To facilitate the operation of fixing a top plate in the closed state by fixing the bottom parts of a pair of stays to the respective stay fixing parts in the bottom part of a main body case while the top plate is closed.

CONSTITUTION: At the time of closing the top plate 4, the bottom edge of a stay 10 is inserted into a guiding groove 7a and turned in a C direction. Then, the front edge part of the stay 10 is lowered along the inclined surface of the guiding groove 7a and guided to the guiding groove 9a of a side plate 9. When the top plate 4 is completely closed, the bottom edge part of the stay 10 is abutted on a bracket 12 and a male screw hole 10a comes to a position almost coincident with a through hole 12a. Then, a bolt 14 is inserted into the through hole 12a from the bottom side of a bracket 12, screwed into the male hole 10a of the stay 10 and fixed to the bracket 12, whereby th top plate 4 can be fixed to a case main body 1 through the stay 10. Thus, the operation of fixing a top plate to the state to be closed can be facilitated.