Then, the step of forming the outer tube 2 having the second lumen 6 communicating the tip to the rear end can be applied to using the same flexible material as that for the inner tube 1, for example, thermoplastic resin such as polyolefin such as polyethylene, polypropylene, ethylene--propylene copolymer and ethylene--vinyl acetate copolymer, polyvinyl chloride, polyamide elastomer, polyester and