The pipes are connected by welding the ends so that they face each other, or, to enhance the assembly, a disc-shaped flange is welded to the end of each pipe, and then the flanges are brought into contact with each other and connected so that they can be separated using bolts and nuts. The ends of pipes with an inner diameter of ∅20 to 50 are plastically processed into a flange shape. This allows for the forming of a flange integral with the pipe, and ensures versatility. When forming a flange on a pipe, it can reduce wear due to friction, improve product quality, and increase the life of the device.