PURPOSE: A hydraulic system is provided to supply constant pressure without the generation of pressure hunting since the supply pressure of a servo valve is kept constant by a reducing valve. CONSTITUTION: A hydraulic system(100) comprises a pump(110), a servo valve(120), first and second hydraulic pipes(210,220), an auxiliary pipe(215) and a reducing valve(115). The servo valve controls fluid pressure. One end of the first hydraulic pipe is communicated with the pump and the other end of the first hydraulic pipe is communicated with the servo valve. One end of the second hydraulic pipe is communicated with the servo valve and the other end of the second hydraulic pipe is communicated with a cylinder(50). One end of the auxiliary pipe is branched from the first hydraulic pipe and the other end lf the auxiliary pipe is communicated with the servo valve. The reducing valve is installed on the auxiliary pipe and controls the operation of the servo valve.