To detect a plate deflection and prevent the plate deflection without being influenced by the ambient environment.

Detecting a rotational speed of an upper roll and a lower roll from the rotational speed of motors 3U and 3L individually, an elongation difference in the longitudinal direction between the upside surface and downside surface of the rolled material L is detected by means of integrating these speed differences. Since the plate deflection of the rolled material L is formed by the elongation difference between the upside surface and the downside, the plate deflection and its degree become possible to be detected based on the distribution conditions of the elongation difference. Consequently, detecting the direction of plate deflection formed toward upside or downside and the degree of its formation based on the distributed conditions of the elongation difference and by means of correcting the rotational speed of motors 3U and 3L in a manner of suppressing the plate deflection, the elongation difference between the upside surface and the downside is suppressed, so that the plate deflection is suppressed.