PURPOSE: To prevent contact of a surface of a case with a working base by a protruding member by providing the member having higher height than that of a member present on the surface of the case on a surface position of the case concealed by coating with a cover.

CONSTITUTION: A protruding member 2 having a higher height than that of a member such as a variable resistor 5, etc., present on the surface of a case 1 is provided at a position on the surface of the case concealed when it is coated with a cover 6. Thus, when a printed board 3 is clamped at the case 1 by screws 4, the member 2 provided on the case 1 is brought into contact with a working base. Since the case 1 is not brought into contact with the base, damage of the surface of the case 1 can be prevented. Since the cover 2 is mounted on the case 1, the member 2 is concealed therein to hold the quality of an external appearance.