Abstract: An abnormal vibration monitor is used for a monitor by detecting the abnormal vibration of a rotary device placed in an explosive atmosphere by using a plurality of piezo-pick-ups. In the monitor, many signal wires and power source wires for devices are placed in an explosive atmosphere whereby there is a possibility of an explosion. In order to prevent the explosion, pre-amplifiers for detection which are respectively connected to the piezo-pick-ups and scanners for scanning the pre-amplifiers and power sources are contained in one antiexplosion casing and the casing is placed in the explosive atmosphere and the parts in the casing are connected by only small number of cables to the monitor placed in an nonexplosive atmosphere. The piezo-pick-ups are respectively fixed to parts for vibration out of the casing to give high detection accuracy.