PURPOSE: To improve corrosion resistance by forming a tube by extrusion- molding a zirconium radical allay at an arbitrary temperature in α area, heating the outside of a tube wall at a temperature in β area and then rapidly cooling it so that an βphase is converted into an α grain structure.

CONSTITUTION: A zirconium radical allay is extrusion-molded at an arbitrary temperature in α area so that a clad tube is farmed. Then, relating to βannealing on the outside of a tube wall before the last cold ralling, only the tube outside wall part is heated at a temperature in β area, then the tube is rapidly cooled to a room temperature. At the β annealing, a protective gas is made to flow in a process chamber so that the outside surface of the tube is not oxidized, and the tube is made to proceed by an induction coil inserted in the process chamber, for heating. In addition, a flushing water is made to flow in the tube so that the inside surface of the tube is kept at a low temperature. Then, a metallic change is caused at the inside part of the tube, and when the tube passes the induction coil, a β phase is converted into an α gain structure where intermetallic bonding particles are finely distributed, so that nodular corrosion resistance is improved.