PURPOSE: To make programming of control program for computer easy by periodically executing such a series of control actions to the elongation percentage of metallic sheet that the elongation percentage of metallic sheet is calculated, a prescribed control operation is executed and operation is outputted in a prescribed cycle.

CONSTITUTION: Operating time (t) is a periodic time that appears in a cycle A independently of the velocity V1 of a steel sheet 3 as a work. Thus, because a dead time T1 becomes a fixed time equal to A, the integration operation times Ti, Tj of a prescribed calculation formula in accordance with integration operation times Ti, Tj become constant values that need not to calculate every operating time (t) and the deterioration of responsiveness in elongation percentage control due to the slowdown of the velocity V1 is eliminated. And, in the case the control part of this invention corresponding to the control part of a rolling device 11 where consists of data output part 9 and signal processing part 10 is constituted of a computer, the programming for elongation percentage control becomes easy and the constitution of the skinpass rolling device is simplified, since the velocity V1 is not necessary to elongation percentage control.