[Task] It should be made possible to control the supply, drain and return flow rate independently of one another for the boom cylinder in a construction machine, comprising a first and a second boom slide valve, each with a first and a second hydraulic pump are connected. [Solution] When the boom cylinder (8) is retracted, the first boom gate valve (16) is configured to control the return flow rate from a head-side oil chamber (8A) to a rod-side oil chamber (8B), the second boom Spool valve (17) configured to control the drain flow rate from a head-side oil chamber (8A) to an oil tank (15), and both the first and second boom spool valves (16, 17) are configured not to drain pressurized oil from the first and second hydraulic pumps (11, 12) to the boom cylinder (8).
展开▼
