PURPOSE: To measure the thickness of a rolled stock with high accuracy without providing a thickness gage in a position adjacent to a rolling mill by providing sheet speedometers respectively in the vicinity of the thickness gage and on the inlet and outlet sides in the vicinity of a rolling roll.

CONSTITUTION: A 1st and 2nd sheet speedometers 11, 12 are provided in the positions adjacent to the rolling mill 1 and on the inlet and outlet sides of the rolling mill 1 and the thickness gage 13 is provided in the position beyond 3m from the inlet side of the rolling mill. Further, a 3rd sheet speedometer 14 is provided in the same position with this thickness gage 13 or in the position at some distance from the rolling mill l to that position. The thickness of the rolled stock P on the inlet side is measured with the thickness gage 13, the moving speed of the rolled stock is measured with the 3rd sheet speedometer 14 and the time when the part for measuring thickness of the rolled stock P reaches the rolling mill 1 is determined. From this thickness of the rolled stock P every moment on the inlet side, the speed Vi on the inlet side and speed Vo on the outlet side of the rolled stock P which is passed through the rolling mill that are measured with the 1st and 2nd sheet speedometers 11, 12, the thickness of the rolled stock P on the outlet side is estimated based on the principle of conservation of mass flow in the rolling mill 1.