PURPOSE: To improve the accuracy of width at the tip and rear end of band steel by successively on-line executing adaptive correction of the linear parame ter of a model formula by using the actual amount of width drop at the tip and rear end with a forgetting coefficient/weighted sequential least square filter.

CONSTITUTION: The weight of the model formula is determined using the fuzzy rule and a short stroke pattern is determined by the weighted moving average method by fuzzy controlling means 10, 11, 12. The width of a band steel on the outlet side of a rough reversing mill is measured with a width measuring means and a width deviation signal is outputted. The amounts of width drop at the tip and rear end and cove are determined by a recognizing means based on a width signal which is obtained by the measuring means. Using the actual amount of width drop at the tip and rear end obtained by the recognizing means, the adaptive correction of the linear parameter of the model formula is successively on-line executed by an adaptive correcting means 9 with use of plural forgetting coefficient/weighted sequential least square filters. In this way, the frequency of rewriting of control data and correction by an operator can be minimized.