PURPOSE: To securely and directly detect the pressure given to an upper die or a lower die by detecting directly the strain in a sensitivity part through a strain gage and detecting the pressure given to the upper die or the lower die based on this detected value.

CONSTITUTION: A hydraulic cylinder 5 is operated suitably to vertically move the upper die 1 and a sheet material is bent by the cooperation of the upper die 1 and the lower die 3. At this time, a strain of the sensitivity part 7 of the lower die 3 is detected actually, this detected value is inputted into a controller 15 through a strain amplifier 17 and an A/D transducer 19 and the pressure given to the sensitivity part 7 based on the detected value is detected. When the detected pressure does not reach a specified pressure, the inner part of the hydraulic cylinder 5 is pressurized through the controller 15 until the specified pressure is detected. Consequently, bending is always performed at a specified pressure.