PURPOSE: To prevent products from lowering in yield and to improve work efficiency by informing a worker that a bending angle is varied, through the change in pressure by which a punch and a die are actuated at the time of the bending of a sheet material, and detecting abnormality before the bending angle is extremely varied.

CONSTITUTION: A pressurizing force to actuate on a ram, which is measured by a pressure sensor at the time of the bending of the sheet material, and a normal pressurizing force at the final processing position of the ram are compared by a comparator part 29 ; whether or not the pressurizing force measured by the pressure sensor 29 is a normal value is confirmed by a confirming part 35 for an abnormal pressurizing force; the worker is informed of the pressurizing force being abnormal by an alarm display part 37 ; and a check for processing conditions is required. Also, if the pressurizing force is not an abnormal value, bending is continued for the sheet material.