Hydraulic control device of construction machine

A hydraulic control device of construction machine, comprising:

• a hydraulic pump;
• a hydraulic actuator to which working oil is supplied from said hydraulic pump;
• an operation member operated by an operator;
• a control valve having a meter-in control flow path lying between said hydraulic pump and said hydraulic actuator, the control valve working so as to change a flow rate of the working oil supplied from said hydraulic pump to said hydraulic actuator through said meter-in control flow path following the operation of said operation member;
• a flow rate control valve lying between a pump discharge line on the upstream side of said control valve and a tank, the flow rate control valve for changing a flow rate of the working oil returned from said pump discharge line to said tank;
• flow rate control means for activating said flow rate control valve for opening and closing in the direction of maintaining a constant differential pressure between a pressure of the working oil in said pump discharge line and a pressure on the downstream side of said meter-in control flow path;
• closed valve maintaining means for maintaining said flow rate control valve in a closed state irrespective of the differential pressure when the pressure on the downstream side of said meter-in control flow path is lower than a set pressure which is preliminarily set;
• a bypass line for communicating with said pump discharge line and said tank on a different route from said flow rate control valve;
• and
• bypass switching means for opening said bypass line when said control valve closes said meter-in control flow path, and decreasing a flow path area of said bypass line following an increase in a flow path area of said meter-in control flow path.