PURPOSE: To make it possible to smoothly process spiral tube in existing tube and to surely diametrically expand the spiral tube so as to bring it nearly close contact with the inner peripheral surface of the existing tube by a method wherein respective side edge parts of bandlike body, which turns to be the spiral tube, are engaged with each other and the spiral tube is produced under the condition that the frictional resistance between the engaged side edge parts is increased by wire.

CONSTITUTION: Band-like body 10 made of synthetic resin is turned into spiral tube 10' by utilizing a tube-making machine 20. The resultant spiral tube 10' is successively introduced in a sewer 81. In the space in the fitting recessed strip 15 of the band-like body 10, which is wound spirally, the inserting part 13b of the fitting projected strip 13 of the band-like body 10, which is newly introduced in the tube-making machine 20. When the inserting part 13b of the fitting projected strip 13 is inserted in the space in the fitting recessed strip 15, the engaging part 13c of the inserting part 13b engages with the engaging part 15a of the fitting recessed strip 15 so as not to come out, resulting in engaging the side edge parts, which are adjacent to each other, of the band-like body 10, with each other. When the tube- making machine 20 is driven under the condition just mentioned above, the thrusting force is given to the bank-like body or spiral tube 10' inserted in the sewer 81 so as to relatively slide the fitting recessed strip 15 of the band-like body against the inserting part 13b of the fitting projected strip 13, which is fitted in the fitting recessed strip 15, resulting in diamet rically expanding the spiral tube 10' successively from the fixed tip side and finally realizing the state that the spiral tube 10' is brought into nearly close contact with the inner peripheral surface of the sewer 81.