A cover member includes: a main body; and a binding member. The main body includes, when viewed from the axial direction, a base area, a first overlap area located on one side, and a second overlap area located on the other side with the base area interposed therebetween. The first overlap area includes a first end part out of the main-body both end parts. The second overlap area includes a second end part out of the main-body both end parts. The base area is formed to be able to change an overlap amount of the first overlap area and the second overlap area. The main-body both end parts are formed from one opening part to the other opening part out of a pair of opening parts that communicate an inside space with the outside.