The present invention has provided therein: a lower rib (first shield section) 69 blocking between a cover 60 and a shield plate 71, below an air intake hole 64; a shield section (second shield section) 72 blocking between a shield plate 61 and a case 11, below an air passage hole 54; and a shield section (third shield section) 73 blocking between the cover 60 and the case 11, above an upper end of the shield plate 71. Airflow flowing in from the air intake hole 64 rises along the shield plate 61 then bends downwards between the upper end of the shield plate 61 and the third shield section 73, drops down along the shield plate 61, and is guided to the air passage hole 54.