A diagnostic apparatus for diagnosising of a valve device in a turbine system comprises at least one of a pressure sensor for sensing hydraulic pressure in a hydraulic cylinder for driving a valve body of the valve device and a position sensor for sensing the displacement of the valve body, and a processing device for processing the output signal from the above-mentioned at least one sensor. The processing device has an offset computing device which receives a signal representing the actual operating state of the valve device obtained on the basis of the output signal from the at least one sensor and a signal corresponding to a reference value obtained under the normal operating state of the valve device. The offset computing device is operable to compute the offset between the signals received thereto. A judging device is provided for judging as to whether or not there is a symptom of a sticking in the valve device, in accordance with the output signal from the offset computing device.