To correctly judge occurrence of abnormality in bearings supporting a main shaft of a pump.
A bearing monitoring device comprises an air supply pipe line 28A, 28B, 28C introducing air, which is supplied from an air compressor 21, in a clearance 38 between non-water pouring bearings 7A, 7B, 7C and the main shaft 5. Flow meters 29A, 29B, 29C detect a flow rate of air flowing through the air supply pipe line 28A, 28B, 28C. A differential pressure gage 33 detects differential pressure between air supply pressure of the air compressor 21 and discharge pressure of a vertical shaft pump 1. A control and judgement section 36 judges occurrence of abnormality in the bearings based on the air flow rate detected by the flow meters 29A-29C and the differential pressure detected by the differential pressure gage 33.