Brackets and are disposed on the back surface of an upper case on which a pair of molded members and constituting a cylindrical structure forming a rear wall of a windproof case are erected. At one end of each of the brackets, the bracket is electrically connected to the molded member and the bracket is electrically connected to the molded member by screws of the upper case, and the brackets are electrically connected to a lower case by screws. Further, a metal plate on the upper case is electrically connected via a metal member fixed to the cylindrical structure.