Abstract: A band-like plate is bent until both ends of the plate meet with each other, and then both ends of the plate are joined by welding so as to make a loop. A pair of first and second inner dies are placed inside the loop while a pair of first and second outer dies are placed outside the loop. The loop is held by the inner and outer dies. At the same time or after that, the loop is pressed by the inner dies when the first and/or second dies move in a given direction.