PURPOSE: To easily straighten the bend at the end part of a steel tube by providing an upper and a lower die having a curvature to correct the spring back of the steel tube in a warm straightening time and a caliber fitting to the outer diameter of the steel tube.

CONSTITUTION: The bend at the end part of the steel tube 1 is measured. A length and a temperature to be heated at the end part of the steel tube 1 are set based on this measured value to heat the end part of the steel tube 1. The spring back at the end part of the hot steel tube 1 is corrected by upper and lower pressure dies having a curvature to correct the spring back in the lengthwise direction and a circular caliber fitting to the outer diameter of the steel tube. In this way, bends based on the residual stress in a cutting time can be restrained from arising.