PURPOSE: To obtain an excellent slip detection accuracy with an inexpensive device structure by performing an alarm output at the time when the peripheral speed difference in the upper work roll and lower work roll of a rolling mill stand exceeds a preset specified value.

CONSTITUTION: The rotary pulse signals P1, P2 transmitted from the pulse generators 9a, 9b provided on the rotary shaft of upper and lower work rolls 4a, 4b is inputted to a slip estimating device 10, which finds a peripheral speed difference by executing an operation. The alarm output of the slip generation is performed at the time when attaining to ≥s by comparing this peripheral speed difference with a preset specified value s. Consequently the generation of a slippage is surely estimated.