PURPOSE: To suppress the non-uniform heating in the thickness direction or the change in the heated range following the change in the tube-making speed, and to improve the quality of tubes and the yield.

CONSTITUTION: Opposite edge parts E on both sides of an open pipe OP to be transported, the first heating device 1 to heat the part E up to the temperature above the Curie temperature, and the second heating device 2 to heat the part E up to the temperature above the melting point are arranged along the transporting direction of the open pipe OP in this order. The position of a coil 4 in the first heating device l is changed forward of or rearward of the transporting direction of the open pipe OP by corresponding to the transporting speed of the open pipe OP, i.e., the tube-making speed.