To perform a cavitation diagnosis with high accuracy without preparing many types of pressure ratio tables in regard to variation of a control valve installation environment (piping layout).

An upstream side fluid pressure Pv1, a downstream side fluid pressure Pv2 and a fluid temperature T of an orifice (contraction flow part) across a valve body in a control valve 101 are detected. The downstream side fluid pressure Pv2 is defined as a fluid pressure at a fluid stagnant part in a flow passage inside the control valve 101. An inner pressure ratio X_Fv(X_Fv=(Pv2-Pv1)/(Pv1-Pv))inside the control valve 101 is calculated in reference to an upstream side fluid pressure Pv1, a downstream side fluid pressure Pv2 and a saturation vapor pressure Pv of fluid calculated from a fluid temperature T. A pressure ratio X_Fv (incipient X_Fvz) inside the control valve 101 when cavitation starts to generate at the control valve 101 is defined as a threshold value X_Fvth, a pressure ratio table TB1 defining a relation between this threshold value X_Fvth and a relative capacity coefficient Cv of the control valve 101 is made and stored in a memory part 100-3. A presence or non-presence of occurrence of cavitation is judged by using this pressure ratio table TB1.