PURPOSE: To easily improve the straightening and the roundness of a steel pipe by providing correcting blocks for executing the same movement as a die while integrally connecting at the front and the rear parts of the die at the prescribed position in plural dies for expanding the pipe disposed as radial.

CONSTITUTION: In the case the bending in the steel pipe 2 is generated, after fitting the correcting blocks 8 to the dies 5 (a table key 13 to be a guide key 7) at a position opposite to bending direction the pipe expansion work is executed. At this time, the correcting blocks 8 are fitted to the dies 5 at the positions 6, 7, 8 or 7 in the case the steel pipe 2 is bent upward. In the case the steel pipe 2 is bent to a right direction, the blocks are fitted to the dies 5 at the positions 9, 10, 11 or 10. Further, in the case the steel pipe 2 is bent to a left direction, the blocks are fitted to the dies 5 at the positions 3, 4, 5 or 4. Further, in the case the steel pipe 2 is bent downward, the blocks are fitted to the dies 5 at the positions 12, 1, 2 or 1.