Hydraulic control system in working machine

A hydraulic control system in a working machine equipped with first, second hydraulic pumps;

• an option hydraulic actuator, which is selectively mounted to the working machine, whose hydraulic supply source is either one or both of the first, second hydraulic pumps;
• and other hydraulic actuators other than the option hydraulic actuator whose hydraulic supply sources are either one or both of the first, second hydraulic pumps,
• wherein the hydraulic control system further comprises first, second option supply oil passages, first, second other hydraulic actuators supply oil passages connected to the first, second hydraulic pumps respectively, acting as pressurized oil supply passages to the option hydraulic actuator, the other hydraulic actuators;
• an option merging oil passage that allows the first, second option supply oil passages to merge thereinto;
• option, other hydraulic actuators control valves that perform oil supply and discharge control to and from the option hydraulic actuator, the other hydraulic actuators respectively;
• and a control device that controls these option, the other hydraulic actuators control valves;
• on the other hand,
• wherein the option merging oil passage comprises the option control valve, and a pressure compensating valve, disposed on an upstream side of the option control valve, that operates in order to maintain a differential pressure, by introducing an inlet side pressure and an outlet side pressure of the option control valve, between the introduced inlet side pressure and outlet side pressure at a predetermined pressure;
• as well as, is configured such that, by connecting a variable relief valve that can vary a relief setting pressure in response to a control signal from the control device, to a signal pressure introduction oil passage that introduces the outlet side pressure of the option control valve to the pressure compensating valve, and lowering a pressure of the signal pressure introduction oil passage down to a relief setting pressure by the variable relief valve and introducing the pressure to the pressure compensating valve, a pressure of the option merging oil passage on the inlet side of the option control valve can be variably controlled by changing the relief setting pressure of the variable relief valve.