PURPOSE: To correct the fluctuation of sheet width properly, by controlling the rolling reduction in width basing on a controlling quantity obtained by adding together the filter-divided outputs of a feed-forward controlling part and a feed- back controlling part.

CONSTITUTION: An output signal H of an arithmetic device 33 itself is used as an output signal J of a controlling part 37 in a case of a gaugemeter feed-back (GM-FB) system, and the output signal J of the controlling part 37 is made to zero in case of a feed-forward (FF) system, and an output signal I of a filter 34 is used as the output signal J of the controlling part 37 in case of a FF+ GM-FB. An arithmetic device 38 adds together a FF controlling quantity corresponding to an output signal of a controlling part 26 and a FB controlling quantity corresponding to an output signal of the controlling part 37, and controls the roll opening of an edger 2 in accordance with an added commanding value by comparing it with a feedback signal of a position detector 16.