PURPOSE: To eliminate a measuring error by detecting a width direction separated distance between a side guide in a state that a material to be rolled does not exist, and a radiation thickness gauge, and determining whether a radiation quantity counting value in this case can be adopted or not, based on a result of said detection.

CONSTITUTION: A thickness gauge 12 executes a reciprocating movement in the width direction in accordance with forward and backward rotations of a motor 21. A pulse generator 26 detects a moving extent in the width direction of the thickness gauge 12, and this detecting signal is inputted to a radiation thickness gauge position detecting circuit 28. Also, the detecting circuit 28 detects a position of the radiation thickness gauge 12 based on this detecting signal, and inputs a result of the detection to a distance operating circuit 20. Subsequently, the distance operating circuit 20 calculates a distance between the inside surface of a recessed part 15c of a side guide body 15a and the center of a radiant ray flux based on an input signal from the detecting circuits 19, 28, and it is inputted to a comparing circuit 30 for determining whether a radiation quantity counting value can be inputted to a thickness gauge computer 70 or not, when there is no plate.