PURPOSE: To secure a certain oil quantity even during a power failure, etc., so as to perform a specified function by constructing a liquid pressure servo device in such a manner that the neutral point of a control valve for outputting a liquid pressure supplied from a liquid pressure source according to a signal quantity from a current amplifier is offset and an output from the current amplifier is also offset.
CONSTITUTION: A liquid servo device supplies pressure oil produced by means of an oil pressure unit 1 to a hydraulic cylinder 2 via an oil pressure accumulator 3 and an electric oil servo valve 4. The electric oil servo valve 4 is controlled by means of a comparator 7 for obtaining a deviation between a detection signal and a control signal from a displacement detector 6 provided in the hydraulic cylinder 2 and a control equipment 8 having an amplifier 5. In this case, the servo valve 4 is adjusted so as to provide the neutral point with a fixed offsetting quantity and the amplifier 5 is so constracted as having an offset for canceling the offset thereof. Thus, during the power failure the servo valve 4 functions as a throttle by offsetting and by securing a fixed oil quantity the hydraulic cylinder 2 can be moved.