PURPOSE: To obtain the subject tube continuously and economically by generating bridging reaction to a thermoplastic tube extruded from an extruder and heating into half melted condition and cooling until crystal precipitates and at the same time a projection is closely contacted to form recessed grooves and cooling the tube rapidly.

CONSTITUTION: Chemical or ionizing radiation bridging reaction is generated on crystalline thermoplastic tube extruded from an extruder and heated by a heat furnace 1 into half melted condition. In position of temperature range from that the resin from the tube 2 begins to cool and crystal precipitates to that all crystal comes out, a contact body 3 having a projection is mounted and closely contacted with tube 2 intensively to form recessed groove and moving the tube in longitudinal direction to form recessed groove on wall surface of the tube sucessively. Condition of close contact between the tube 2 and contact body 3 can be adjusted by a roller 5 supported on a support 4. Helical groove can be made by rotating contact body.