PURPOSE: To stabilize a system even when a load pressure difference is zero, and prevent a waveform variation by eliminating looseness of the system.

CONSTITUTION: In a hydraulic servo system provided with a controller 1 to make an object 8 follow a command signal value, an electric current amplifier 2 to amplify a signal from the controller 1, a servo valve 3 and a servo cylinder 6 to brake the object 8 by a signal of the electric current amplifier 2, the hydraulic servo system is provided with a sub-electric current amplifier 10 to amplify the signal from the controller 1, a sub-servo valve 11 and a sub-servo cylinder 12 to brake the object 8 by a signal of the sub electric current amplifier 10.