To reliably and continuously detect rotational abnormality, particularly, a nonrotating condition in a nondriven roller by detecting rotational abnormality of the roller on the basis of a vibration signal of a roller support part.

It is arranged in a housing of a support roller in a plating line pot, and detects vibrational acceleration in a feed plate. A detected signal is amplified by a vibration amplifier 4, and is outputted to an active band-pass filter 5. On the other hand, operation is performed on a roller rotating frequency from a line speed signal and a roller diameter inputted from an external part by an operation circuit 6, and a central frequency of the active band-pass filter is determined. The active band-pass filter 5 varies a filtering band by this central frequency, and extracts only the roller rotating frequency. A signal level is monitored by a judging circuit 7, and when it exceeds a threshold value, an alarm is outputted by an alarm device 8. Therefore, an alarm is outputted at a high detecting rate, and an unrotating condition is detected in its early stages.