A spring washer sandwiched between flat washers is interposed between a land on one surface of a printed circuit board of a control board part and an attachment plate, and a screw is inserted from the other surface of the printed circuit board through a substrate through hole extending through the printed circuit board, one of the flat washers, the spring washer, and the other flat washer and is screwed into a screw hole formed in the attachment plate, thereby fixing the attachment plate to the printed circuit board.