PURPOSE: To obtain a steel plate excellent in flatness and that of uniform quality, by cooling a material to be rolled while introducing the center line of it to that of a coolant.

CONSTITUTION: A width detector 19 dispose at the exit of a rolling mill detects the width of a finished plate to transmit its signal to a calculating device 20. The device 20 calculates the setting width of side guides 14, 14' and the timing of setting of them basing on both of further inputting signals from a material information 21 referring to the thickness and components and from tracking information 22, to transmit a signal to an automatic position controlling device 23 thereby outputting to a side-guide driving device 24 to drive the guides 14, 14' for setting them. By this setting, the center line of a material 3 to be rolled is automatically aligned with that of the coolant. Shielding plates 4, 4' disposed at the coolant are used to prevent the edge parts of the material from cooling and to cool the center part of it.