PURPOSE: To improve the bending accuracy and to prevent the work flaws from being generated by providing an image pickup means to pick up the image of a marker part equipped in the pressurizing direction of both an upper die and a lower die and a marker position detecting means, detecting the deviation of the dies automatically and correcting the amount of the deviation.

CONSTITUTION: When the work W between a punch 13 and a die 9 is bent by the relative motion of both dies, the images of the marker parts 27, 29 provided in the pressurizing direction of both dies are picked up by a visual sensor 17. The position of each marker of the marker parts 27, 29 is detected by an image processor 19 based on this image pickup. When the position of each of detected markers is displayed on a display 23, an operator corrects the deviation of the die manually based on the display. Otherwise, a die deviation correcting actuator is operated to correct the die deviation. Consequently, bad influence by the unbalance load to the frame can be removed.