PURPOSE: To detect bent of a rolled member continuously in real time at high accuracy.

CONSTITUTION: The central position of a band steel, being calculated by a first sensor 1 at a predetermined interval, is tracked based on the speeds of the band steel detected by the first sensor 1 for operating the center of the rolled band steel based on the width thereof and a speed sensor 3 disposed at same position. Every time when the central position of the band steel reaches the position of a second sensor similar to the first sensor 1 and spaced apart therefrom, the center of each band steel is calculated based on the width of band steel detected by the first and second sensors 1, 4. A computor 5 then converts the shift of the center of each band steel into a vector which is jointed one by one thus detecting the bent of a board.