PURPOSE: To detect the oil leakage without unnecessarily increasing the amount of allowance oil, by generating a second oil leakage warning signal by measuring the amount of oil flowing in the calbe after a first oil leakage warning is generated by a compound gage having a contact point.

CONSTITUTION: A pointer 30 of the compound gage 18 having the contact point is contacted with the contact point 32 at an oil pressure point Pa, and the first oil leakage warning signal is generated. A warning device is operated and the first oil leakage warning is generated and the signal is supplied to a logic circuit 22. When the logic circuit 22 receives this signal, the measurement by an oil gage 20 is made to start. The oil gage 20 measures the amount of the oil flowing in the oil-filled cable 12 from a pressure oil tank 14, generates an electric signal, and supplies the signal to the logic circuit 22. The logic circuit 22 generates the second oil leakage warning signal based on said electric signal when the amount of the oil V1 corresponding to a specified amount of the oil, i.e. the pressure difference between Pa and Pm is attained.