PURPOSE: To perform cyclic driving of the lateral by stopping a movement of the faster lateral, waiting for the slower lateral to catch up with the faster one, and redriving the slower lateral when a difference between a working side and a driving side lateral movement amount reaches an upper limit in the case that the above difference occurs.

CONSTITUTION: A controller 7 starts its calculation by a lateral concurrently driving command. Solenoid valves SV-1, SV-2 are energized to drive upper and lower intermediate rolls 3, 4. In the case that a moving speed of the working side lateral is faster, the SV-1 is demagnetized and the working side lateral is stopped to wait for the driving side lateral to catch up with the former lateral. If the driving side lateral is faster. The SV-2 is demagnetized and the driving side lateral is stopped to wait for the working side lateral to catch up with the driving side one. Thus, both the working side and a driving side lateral are concurrently driven to stabilize the rolling.