PURPOSE: To realize prevention of cambering of a rolling material by detecting directly amount of deviation of the rolling material in the entrance side of a rolling mill utilizing light emitted from the rolling material and adjusting right and left rolling reduction until the amount becomes zero.

CONSTITUTION: Right and left gaps of upper and lower working rolls 1, 2 of a hydraulically loaded rolling mill that rolls a heated rolling material 9 are set by controlling the quantity of pressure oil that flows in and out to and from right and left hydraulic cylinders 7, 8 by servo values 11, 12. At the same time, movement of pistons of cylinders 7, 8 is detected by displacement detectors 13, 14, and their signals are compared with set signals by summing amplifiers 15, 16, and if difference is found, corrected by adjusting the valves 11, 12. Detectors 17 that detect position of end of width basing on light emitted from the rolling material 9 are provided right and left of entrance side of the rolling mill, and difference between the signals, that is, amount of snaking of the rolling material 9 is determined by an arithmetic unit 18, and this is compared with a target signal of a setting device 19 by an arithmetic unit 20. The amount of cambering deviation signal 22 is processed by a control regulator 21, and right and left draft correction signals 23, 24 are added to amplifiers 15, 16.