PURPOSE: To suppress the occurrence of dished or domed camber produce at the tip of material, by changing over a speed balance circuit in engaging to a load balance circuit after engaging.

CONSTITUTION: A speed balance control circuit 13 in engaging is composed of circuits 21 and 22 differentiating the speed feedback values of upper and lower rolls 2 and 3, a circuit 23 checking that differentiated values and subtracting, and a circuit 24 to convert the speed deviation into the current equivalent. A change-over circuit 25 changes over the speed balance control circuit 13 in engaging to a load balance circuit 10 after engaging. On this account, the unpredictable speed difference can be suppressed with high response which is produced between upper and lower rolls in engaging with material, so that the dished or domed camber can be suppressed that occurs at the tip of the material.