Abstract: An upper cover (12) and a lower cover (14) are fastened to each other on their front faces by a hexagon socket head cap screw (40). While a cover supporting column (34) is received on a column seat (38) of each fixation tongue (30) projected from the upper cover (12), the end face of the cover supporting column (34) is brought into contact with a column contact surface (36) of the fixation tongue (30). The hexagon socket head cap screw (40) is inserted through a spot facing slot (60) and a through slot (58) of the lower cover (14) and a through hole (48) of the upper cover (12) and eventually screwed into a tapped hole (42) of the cover supporting column (34). The cover supporting column (34) and the fixation tongue (30) of the upper cover (12) are positioned and fixed to each other via the hexagon socket head cap screw (40), which goes through the positioning through hole (48) of the fixation tongue (30) and is screwed into the tapped hole (42).