A yoke, as embodied, includes a base having a center vent; at least a pair of inner yoke portions respectively located at both end edges of the center vent and protruding from the base to face each other, each of the inner yoke portions having a support protrusion, the support protrusions being respectively located at complementary positions of leading ends of the inner yoke portions; and at least a pair of outer yoke portions protruding from the base and facing each other.