PURPOSE: To concurrently drive the working side and the driving side lateral and to stabilize rolling by concurrently driving the working side and the driving side lateral and repeating driving and stopping of a faster lateral every a constant time when a difference between both the lateral movements occurs.

CONSTITUTION: A controller 7 starts its calculation and solenoid valves SV-1, SV-2 are energized to drive upper and lower intermediate rolls 3, 4 by a lateral concurrently driving command. A decision on the relative magnitude of moving speeds of V1 and V2 is made. If V1=V2, energization of the SV-1 and SV-2 is continued. If V1>V2, that is, the moving speed of the working side lateral is larger than that of the other side, energization and demagnetization of the SV-1 are repeated every a constant time to reduce the V1 down to the lateral moving speed V2 of the driving side. Then, continuance or discontinuance of the calculation is decided: in the case of continuance, the operation returns to a detection process of the V1 and V2 and in the case of discontinuance, the operation is terminated.