To restrain and prevent the occurrence of slippage between rolls by providing a detecting means for detecting the slippage between the rolls with a pulse counter detected by a pulse detecting means.

When the pulse counter 6 count-integrates the pulse outputted from a work roll fitting PGW 5 and inputs a backup roll pulse PGB 2, the count value PWI is outputted to an arithmetic circuit 9 and at the same time, the count value is reset and the count integration of the PWI of the PGW outputted pulse is again repeatedly executed from zero. The arithmetic circuit 9 outputs the slippage ratio R between the rolls to a comparator 10 and a comparator 13. The comparator 10 compares an allowable slippage ratio R1 and the slippage ratio R between the rolls outputted from the arithmetic circuit 9, and when the R is larger than the allowable ratio R1, the allowable excessing signal is outputted to an alarm generator 12 to call the countermeasure for reducing the slippage between the rolls.