PURPOSE: To manufacture a steel plate small in plate thickness deviation by connecting a mill constant to a value near a real value in an automatic plate thickness control.

CONSTITUTION: The roll gap S of a work roll 2 is measured by a load F driving rolling by a load cell 4 and the position detector 6 of a cylinder 5, a rolled load deviation ΔF and a roll gap deviation ΔS are found and a plate deviation Δh is calculated from a gage meter equation: Δh=+ΔS+ΔF/K including the mill constant K. On the other hand, a deviation {Δh(γ)} from the target plate thickness is measured from the plate thickness (h) measured by a plate thickness meter 14, the mill constant is calculated inversely from an operated plate thickness deviation Δ(Δh) and a measured plate deviation Δ{Δh(γ)}, the mill constant obtained by such an inverse operation is used as the mill constant of the following pass or the following stand.