To block the outflow of a foreign matter from the inside of an angle part even when a jacket layer in the angle part is damaged and to surely detect the presence/absence of the damage of the jacket layer or the like by an airtightness test.

The jacket layer 14 constituting the angle part 3b is made to the double structure of an inner layer 14a and an outer layer 14b, and the foreign matter inside the angle part 3b is prevented from leaking out even when the outer layer 14b is damaged. Also, between the inner layer 14a and the outer layer 14b, as a pressure leak-out part for making pressure leak out between the inner layer 14a and the outer layer 14b when the pressure is introduced to the inside of the angle part 3b, a wire 20 is inserted between the outer surface of a base end ring 10b and the inner layer 14a of the jacket layer 14, and the communicative path of air is formed with the wire 20.