PURPOSE: To properly hold the contacting face pressure between an end face of roll and side dam and to prevent the molten metal leakage by calculating a difference between an opening rate corrected by deducting a thermal expansion quantity at the shaft direction of roll from the opening rate of side dams and the prescribed value and controlling, it so as to become to the prescribed value.

CONSTITUTION: At the time of casting, a signal (x) after correcting to the opening rate of side dams continuously by an arithmetic unit 13 based on signals x1, x2, detected by position detectors 6, 12, is transmitted to an arithmetic control device 14 to compare with the prescribed value xa. In case of Δx=x-xa>0 as using Δx for a signal of deviation value, the opening rate of side dam 3 exceeds the prescribed value. Therefore, a signal corresponding to the deviation value Δx is transmitted to a pressure control device 15 from the computing control device 14, and hydraulic pressure Δp is increased from the pressure control device 15 to decrease the opening rate of side dams 3. At the time of becoming to zero for the deviation value Δx, the signal from the position detector 6 becomes to x1, and is fed back to the pressure control device 15, to hold the pressure hereat, and in case of becoming to negative caused by overpressure, the pressure is decreased.