PURPOSE: To prevent malfunction of the device and to rapidly detect a bite stop state by respectively comparing torque and load changing amounts with deviation comparing fixed values and detecting a bite stop state of a rolling mill by a comparator for load-on detection.

CONSTITUTION: Electric voltage, electric current detectors and a coefficient multiplier 20W22 of a rolling mill are installed; a comparator 6 for load-on detection and a former torque value hold circuit 26 are also installed. A rolling mill torque is calculated based on respective detection values from the detectors 20W22 and a torque changing amount is calculated through the hold circuit 26 and a subtracter 27. Then, a load changing amount is detected by a former load value hold circuit 30 and a bite stop state is detected by comparing those values with deviation comparison fixed values, respectively. At last, the bite stop state is conformed through the comparator 6. Malfunction of the device is prevented and detection of a bite stop state is rapidly detected because detection of the state through three types of detector and comparator is performed.