A press brake has a lower table arranged at lower part of left and right side frames, an upper table arranged at upper part of the side frames and facing the lower table, one of the upper and lower tables serving as a ram to be moved vertically, and left and right vertical driving units arranged on the side frames, respectively, to move the ram vertically. Strain detecting sensors are arranged on inner and outer side faces, respectively, of each of the left and right side frames, to detect strain of the side frames in such a way as to cancel horizontal deflection of the side frames and detect vertical deflection thereof.