PURPOSE: To ensure that a block in which an oil leakage is generated can be separated from a hydraulic system and a steering function can be safely maintained by the remaining hydraulic system, by controlling ON-OFF valves in response to an oil leakage signal.

CONSTITUTION: For example, when an oil is leaked from a pipe 3 at a point C, a reflective wave is generated in an oil leakage detecting cable 8c, a controlling panel 15 judges that the oil is leaked from a block E, and outputs a controlling signal for closing ON-OFF valves 12W14. By this, the block E is disconnected from the hydraulic system, and a pump P1 is simultaneously stopped. A rudder 6 is rotated without any trouble since a pressure oil from a pump P2 is received by one of cylinders 4a, 5a and is discharged through the other. Accordingly, the steering function can be maintained.