To provide a control system of a hydraulic actuator which is capable of suppressing, e.g. behaviors of the prescribed state variables of the control system such as spool displacement of a servo valve and hydraulic cylinder differential pressure, and contriving the high response of a hydraulic actuator.

In a control system driving a motor driving circuit 7 and a spool driving motor 8 by a control unit v from a controller 6 in which a cylinder stroke x of a hydraulic cylinder 3 and the desired value r are inputted, displacing a spool 2a of a servo valve 2 controlling hydraulics supplied to the cylinder chambers 31 and 32 of the hydraulic cylinder,3 and performing a feedback control of the cylinder stroke of the hydraulic cylinder 3 based on an optimum control theory, a compensator minimizing a performance function in which arithmetic values of state variables to be suppressed such as arithmetic values of estimated hydraulic cylinder differential pressure calculated by using a controlled system characteristic or estimated servo valve spool displacement, for instance, are fetched is provided.