First, the pressure of oil within an oil passage in a control valve device is measured. Then, an amplitude and period of pressure fluctuations are detected on the basis of an obtained measurement result, and an oil vibration state or an oil non-vibration state is diagnosed. In this case, when the amplitude is greater than a reference amplitude value and when the period is less than a reference period value, the oil vibration state is diagnosed. Further, a duration time of the oil vibration state is determined on the basis of a diagnostic result. Then, when the oil vibration state continues for a period greater than or equal to a reference time, warning information is output. In this way, warning information is output stepwise in accordance with occurrence or continuation of oil vibration.