To prevent a thick plate after straightening from generating a defective part due to a large strain by previously setting a non-control zone specified in accordance with the strain quantity of the thick plate before straightening within the dangerous load zone determined in view of equipment restriction in the case of executing compensating control of the extending quantity of a leveler for the load generating at the time of straightening.

An upper frame 4 is descended with an electrically-driven screw down device 7 at the time of making the plate pass, and the thick steel plate 2 is straightened while bending repeatedly by the initial inter meshing of upper and lower rolls 3. At the time of straightening, compensating control of the leveler extending quantity is executed with a controller 11 until the measured load value obtained with a pressure cell 10 enters the non-control zone. When an excessive load due to the end part camber of the thick steel plate 2 is generated and the measured load value enters the non-control zone, compensating control of the leveler extending quantity is suspended to restrain the straightening load with the controller 11. Further, when the measured load value exceeds the dangerous load zone at the time of suspending compensating control, the load by a hydraulic reduction cylinder 8 is released with the controller 11.