PURPOSE: To enhance the modification accuracy of a standard surface by measuring and storing the warp quantity of a section generating in a downward projected form and correcting the warp quantity, measured by inverting the section by 180°, based on the stored value and modifying the section based on the corrected value.

CONSTITUTION: A steel section 1 is put on supporting posts 2a, 2b with its standard surface 1a faced downward, and its bent quantity δ with respect to a zero point 7 is measured by a measuring device 3 to store the results in an arithmetic unit. When the warp of sectional is downward projected, the section 1 is inverted by 180° to make its surface 1a face upward, and a modification value δ2=δ1-δ is obtained by measuring the warp quantity δ1. Next, a ram 4 is lowered, to correct the position of surface 1a so as to set it to the point at a distance δ2 from the zero point 7. In this way, the warp of section 1 of the standard surface side is modified to be within a range of specifications, even when the sectional has non-parallel upper and lower faces or recessing grooves.