PURPOSE: To practice a controlling work so as to obtain a sheet thickness having very high accuracy, by installing a controller for correctively controlling the roll gap against the variation of the sheet thickness on the run-out side of a rolling mill be caused by said variation on the feed-in side.

CONSTITUTION: A gain operation device 19 receives following inputs; reference sheet thickness HSmm on the feed-in side; controlled mill constant Mkg/mm; sheet thickness deviation ΔHFmm for correcting the gain and lock-on value ΔHOmm for sheet thickness deviation on the feed-in side, respectively; lock-on value POkg for the rolling load; rolling load deviation ΔPFkg for correcting the gain; lock-on value SOmm for the roll gap; and roll gap deviation ΔSFmm for correcting the gain. From these data, the sheet thickness Hmm and (h)mm on the feed-in plus the run-out sides at present time are respectively operated. Next, a plasticity factor (m)kg/mm, and then, a correcting value ΔSFGmm for the aimed roll gap are operated. The device 19 outputs this correcting value ΔSFG which is then added to the reference roll gap SRmm and their sum as the aimed roll gap SPmm is inputted to a screw position controlling system 6, therefore the roll gap S is controlled.