An intermediate holding member is inserted between a second free end and a third free end from an inner circumferential side in a radial direction, a first radial restraining member presses against a free end at an innermost circumference from radially inside, and a second radial restraining member presses against a free end at an outermost circumference from radially outside. Next, a voltage is supplied between a torch and the members, an inert gas is supplied to the torch, and an arc discharge is generated between the torch and the free ends, welding the radially-adjacent free ends.