To provide an erosion prediction method and an erosion prediction device of a hydraulic machine capable of easily predicting a place of an inner flow passage of the hydraulic machine where erosion by cavitations occurs and an erosion speed.

In the erosion prediction device of the hydraulic machine for predicting erosion occurring by the cavitation in the inner flow passage of the hydraulic machine such as a pump, a plurality of piezoelectric elements 14 for converting a shock load of the cavitations into a voltage signal are provided in a surface of the inner flow passage 13 of an actual hydraulic machine or a model hydraulic machine, a light emitting element 15 for converting a voltage signal emitted by the piezoelectric elements 14 into light is provided at a predetermined position of the hydraulic machine and an erosion area-erosion quantity prediction means for measuring light emitted by the light emitting element 15 through observation windows 5 arranged in a bell-mouth 2 and a pump casing 4 when cavitation occurs and predicting an area where erosion occurs and/or an erosion quantity caused by the cavitation in the inner part passage 13.