The step of forming inner tube 1 having the first lumen 4 communicating from the tip to the rear end can be conducted using flexible material, 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 polyurethane, silicon rubber or latex rubber