PURPOSE: To accurately detect the elongation rate of a plate material when the plate material is subjected to bending.

CONSTITUTION: The elongation rate of the plate material generated by bending the material at a prescribed angle is detected by a stage for obtaining by arithmetic processing the approximate linear expression of the end faces in respective bent flange parts a1, a2, a stage for determining the intersected point of the linear expression of the end face in the base part a3 formed between a pair of the bent flange parts a1, a2 and the approximate linear expression of the end faces in the respective bent flange parts a1, a2 and also detected by arithmetic processing for obtaining the length at the end face of the base part a3 from this intersected point and a stage arithmetic processing for obtaining the difference between the length at the end face in the base part a3 and the length at the end face in the base part a3 when assumption is made that the elongation does not arise.