PURPOSE: To detect the configuration continuously and to perform control with excellent response by changing the exciting method of a plurality of magnetic sensors which are arranged in the plate width direction of the belt shaped rolling material in accordance with the rolling status.

CONSTITUTION: A rolling state judging circuit 16 judges the sensor exciting method. A constant exciting command (d) for the specific sensor is sent to a timing signal generating circuit 20 and an exciting sensor jusging circuit 18 at the time of nonsteady rolling when the rolling speed is slow, A sequential exciting command (e) is sent to the circuits 20 and 18 at the time of steady rolling when the rolling speed is high. The circuit 18 sends a exciting sensor designating signal (f) to the timing signal generating circuit 20 based on the signals (d) and (e). Then an ON-OFF signal (b) corresponding to the designated mangentic sensor is always turned ON and the other signals (b) are always turned OFF. Based on said signal (b), only the designated sensor is always excited by power source control circuits 22-1W22-a. Based on their outputs, the configuration is continuously computed by a configuration computing curcuit 24 at every sampling period. Based on its output, the control with excellent response is preformed.