PURPOSE: To execute a continuous operation when changing a plate thickness, and to improve a yield by adjusting most suitably a rolling reduction of a breakdown stand in accordance with a variation of the plate thickness.

CONSTITUTION: A plate thickness of a steel band is measured by a plate thickness meter 12 installed at the entry of a breakdown stand, and in accordance with a difference of a measured value and a measured value of an initializing time, a controller 11 operates a correcting quantity of an inter-roll gap of each breakdown stand. A result of said operation is outputted as a control signal to a rolling reduction device having rolling-reduction motors 14aW14d... for adjusting the inter-roll gap, and the rolling reduction reduction motors 14aW14d... are controlled. A controlled variable of each stand is detected by detectors 15aW15d... of a selsyn counter etc., and its information is fed back to an operator 11.