A performance monitoring apparatus for a fluid machinery which includes a predicted performance curve calculator for obtaining a curve representing the relationship between a pressure coefficient and a flow coefficient by non-dimensional characteristics per a plural fluid control quantities from a compression ratio or a pressure difference and an inlet flow rate of the fluid machinery, and a performance monitoring calculator for obtaining an actual performance head from fluid control quantities, a suction pressure, a discharge pressure, a suction temperature, a compression coefficient, a gas average molecular weight, and a specific heat ratio at the time of the operating fluid machinery, and obtaining a predicted performance head from a predicted performance curve, fluid control quantities, and an inlet flow rate; and calculating a performance degradation from the ratio of the predicted performance head to the actual performance head.