An upper lid and a bottom lid are fastened to flange portions formed in outer side surfaces of a casing main body. A metal collar to which a screw serving as pressing means (a fastening member) is inserted is embedded in each flange portion. Both end portions of each of the metal collars protrude at a predetermined length toward the upper lid and the bottom lid from the flange portion. A predetermined interval corresponding to the length of the protruding portion of the metal collar is created between the casing main body and the upper lid and between the casing main body and the bottom lid, by bringing the upper lid and the bottom lid into contact with the protruding portions of each of the metal collars at a time of assembling.