PURPOSE: To facilitate attachment while decreasing the number of screwing points by molding a hook part in one body at a proper place on the internal flank of a housing cover, and engaging the hook part with a main body frame and fitting the housing cover to the main body frame.

CONSTITUTION: The hook part 3 of the housing cover 1 is engaged with the L- shaped edge of the main frame 6, and the cover is fixed to the L-shaped edge of the main body frame 6 by inserting screws into screw holes 5 formed in a support plate near upper inside corners. Consequently, the housing cover 1 is at the lower part on the edge of the main body frame 6 with the hook part 3 and never falls; and the support plate arranged near two corners of the upper inside is fitted with screws the obtain fixation in a nearly complete state.