A diagnosis method is provided for appropriately predicting a state change of a valve with a simple configuration. The valve diagnosis method comprises: calculating a value of a first rate of change, with respect to a predetermined damage index during reference data acquisition, of the predetermined damage index during diagnosis; calculating a value of a second rate of change, with respect to a root mean square value calculated from a difference between predicted waveform data of vibration during reference data acquisition and vibration data serving as reference, of a root mean square value calculated from a difference between predicted waveform data of vibration during diagnosis and vibration data during diagnosis; in a plane, arranging a plot of a diagnostic result based on a value of the first rate of change being calculated and a value of the second rate of change being calculated; and determining a state of a valve to be diagnosed based on a value of the first rate of change being calculated, a value of the second rate of change being calculated, and an arrangement position on the plane of a plot of the diagnostic result.