To excellent prevent a slip between a deflector roll and a metal plate by imparting an exact speed command and acceleration/deceleration compensation without delay to the deflector roll.

By correcting, for example, the actual speed VPV of the metal plate detected with a speedometer (not shown in figure) provided on the outlet side of a rolling mill 1 based on thickness (t) of the metal plate 3 and roll radius (r) of the deflector roll 4, and exact speed command VSV is outputted to a speed controller 8 with a speed command outputting device 6. This speed command VSV is determined by VSV=VPV×r/(r+(t/2)). By calculating the timewise change rate dV/dt of the speed command V of the rolling mill 1 with an acceleration/deceleration compensating circuit 11, lead speed corresponding to the level of an acceleration/deceleration rate is impartted to the deflector roll 4 and the deviation of steady speed during acceleration/deceleration is removed and a current command for correcting a torque reference T so as to secure speed matching property with the speed of the rolling mill 1 is outputted.