Hydraulic drive system of construction machine

A hydraulic drive system of a construction machine, comprising:

• a turning motor and a boom cylinder, each of which serves as a hydraulic actuator;
• a first hydraulic pump and a second hydraulic pump, whose respective tilting angles are adjustable independently of each other, each pump discharging hydraulic oil at a flow rate corresponding to the tilting angle of the pump;
• a turning control valve for controlling supply of the hydraulic oil to the turning motor, the turning control valve being disposed on a first bleed line extending from the first hydraulic pump;
• a boom main control valve and a boom auxiliary control valve each for controlling supply of the hydraulic oil to the boom cylinder, the boom main control valve being disposed on a second bleed line extending from the second hydraulic pump, the boom auxiliary control valve being disposed on the first bleed line;
• a turning operation valve that outputs a pilot pressure to the turning control valve;
• a boom operation valve that outputs a pilot pressure to the boom main control valve;
• and
• a boom-side regulating valve that outputs a pilot pressure to the boom auxiliary control valve in accordance with a boom raising operation when no turning operation is performed, and outputs no pilot pressure to the boom auxiliary control valve when a turning operation and a boom raising operation are performed concurrently.