An angle detecting head provided with an emitter and a pair of receivers is rocked, and a bent surface of a workpiece bent by a punch and a die of a bending machine is irradiated with a measurement light from the emitter, a reflected light is received by the pair of receivers, and a bending angle of the workpiece is obtained from a peak value of the light received by the respective receivers. The angle detecting head is movable parallel to the die, is movable in a direction close to and away from the workpiece, and is movable in a direction in which the die is raised/lowered. The angle detecting head is positioned in an optimum angle measurement position of the workpiece and the bending angle is measured. Moreover, the bending machine presses the workpiece to the vicinity of a target angle, measures the bending angle of the workpiece during final pressing, removes pressure to bring the workpiece to an unloaded state, measures the bending angle of the workpiece, and obtains and stores a difference between the final pressing angle measured value and the unloaded angle measured value as a spring-back amount.