PURPOSE: To improve accuracy by calculating a deviation signal of a thickness meter over the entire length of a rolled stock, variation of the gage plate thickness at a final rolling stand, a signal obtained by trucking the rolled stock as far as the installed position of the thickness meter, etc., to obtain a mill rigidity coefficient.

CONSTITUTION: In an expression I, mL maximizing R(0) in τ=0 is estimated as the mill rigidity coefficient in a final stand. hx(t): deviation signal of a thickness meter over the entire length of the rolled stock, hL(t): variation of gage plate thickness at the final rolling stand, R(τ): mutual correlative coefficient of a signal, TL: rolling time in the final rolling stand. In the expression II, the variation H of the plate thickness on the inlet side is obtained. P: load variation at the rolling stand, S: variation of a drawing down position, M: estimated mill rigidity coefficient, Q: coefficient of plasticity, P/ H: influence coefficient, a suffix indicates a stand number. In an expression III, M maximizing R in τ=0 is estimated as a mill rigidity coefficient in an (i)-th rolling stand. Ti: rolling time in the (i)-th rolling stand.