PURPOSE: To simplify production stages and to improve appearance grade without shaking after assembly of upper and lower cases by preventing mating misalignment in assembly of the cases and forming welding surfaces near to the outside surfaces of the cases, thereby making welding easy and sure.

CONSTITUTION: Either of the upper and lower cases 1, 2 is provided welding bosses at plural points. Fitting holes 4 to be inserted with these welding bosses 3 and caulking receiving holes 5 for welding, bulging and solidifying the front ends of the welding bosses are disposed on the other case opposite thereto. As a result, the troubles in the assembly stages are decreased and the mating misalignment and flawing of the upper and lower cases are prevented. The assembly operation is thus simplified, the generation of the spacing and misalignment between the upper and lower cases is prevented and the quality is improved.