PURPOSE: To prevent the abnormal rising of secondary pressure or the like by making the set pressure, generated by a spring, of a secondary control valve variable through solenoid electromagnetic force, and forming the secondary pressure in the method of a linearly controlled proportional electromagnetic relief valve.

CONSTITUTION: The electromagnetic force of a proportional solenoid 54 acts onto a spool 52 of a secondary control valve 50 according to spring force of a spring 53, hydraulic reaction force of secondary pressure Ps and solenoid current Is, and makes set pressure generated by a spring 52 variable, and the secondary pressure Ps to a secondary cylinder 24 is linearly controlled by the solenoid current. An electronic control unit 70 establishes target secondary pressure, and supplies the solenoid current Is corresponding to the above pressure to make the secondary control valve 50 control the secondary pressure Ps, and moreover establishes primary pressure Pp through a primary control valve 60. Thus since this controller is formed in a method by which pump discharge pressure is relieved for pressure regulation, troubles such as the abnormal rising of the secondary pressure or the like can be prevented.