At a time of executing a bending process of a work according to a cooperation between a punch and a die, a provisional stroke value for obtaining a predetermined angle is determined on the basis of a bending process condition. A test bending process is executed on the basis of the provisional stroke value, and a striking number required for striking the work until the predetermined angle is obtained is judged. Since a stroke value correction amount at a time of a main bending corresponding to the striking number and the predetermined angle is previously determined, a main bending process is executed by correcting to a stroke value at a time of the main bending by computing the corresponding stroke value correction amount on the basis of the stroke value correction amount at a time of the main bending process.