PURPOSE: To surely straighten a base metal by forming a step difference chute hole of predetermined size halfway a pipe-shaped guide path and permitting the base metal, placed in the same direction to the direction of shape of the chute hole, to drop in the hole while the base metal, placed in the reverse direction, to pass over the hole.

CONSTITUTION: A pipe-shaped guide path 10 provides in its bottom halfway the path 10 a chute hole 13 to be drilled. This chute hole 13 forms its part 12 into a rectangular hole having a width a little larger than the diameter in a small contour part 2 of a base metal 3 and a length almost equal to the length of the small contour part 2. While forming a part 11 into a rectangular hole having a length almost 1/2 times the length of a large contour part 1 of the base metal 3 and a width a little larger than the diameter of the large contour part 1, the part 11 is formed being continued to the part 12. In this constitution, advancing the base metal 3 with its small contour part 2 leading, it is first placed advancing into the part 12 of the chute hole 13, successively advancing the large contour part 1 to pass through the chute hole part 11, the base metal 3 drops down. However, the base metal, placed in the reverse direction, is unable to drop down from the hole parts 11, 12, passing over being left as placed in the reverse direction. Thus the base metal can be surely straightened.