PURPOSE: To detect a spring back angle of work in a high precision at the step when a work is not completely unloaded.

CONSTITUTION: A bent angle θ1 of work is detected with descending an upper die down to unloading start position, next, with ascending the upper die in slow speed, an elevating position of the upper die and a press load applied on work are sucessively detected at ascending. And an increased quantity of forming force per an unit ascending distance of the upper die is sucessively computed and at the point thereafter where the increased quantity value is converged to the fixed value, a bent angle of work is detected, based on each data of the bent angle and forming force, the relationship of bent angle vs upper die position and relationship of forming force vs upper die position are primarily approximated, with computing the upper die position where the forming becomes zero by the approximate equation, a bent angle θ4 of the work 13 at the upper die position is assumed. The difference θ4-θ1 between the bent angle θ4 and bent angle θ1 at unloading start position is spring back angle Δθ.