To permit smooth performance of both of an operation requiring a high pressure and an operation desired to produce a pressure at a suppressed level, a shuttle block 50 positioned between pilot operating units 35-37 and flow control valves 5-15 and pump regulators 28a,28b is arranged in association with shuttle valves 61-63,65-75, which select maximum pressures of groups of operation signal pressures produced by the pilot operating units 35-37, respectively, and at least one of the plural groups of operation signals, and is constructed to include hydraulic selector valves 81,82 and a boom-lowering, hydraulic selector valve 83. The hydraulic selector valves 81,82 are operated based on the maximum pressures to produce control signal pressures from a pressure of a pilot pump 2. The boom-lowering, hydraulic selector valve 83 is operated based on an operation signal pressure Dd relating to a single boom-lowering operation among the operation signal pressures produced by the pilot operating units 35-37 to produce a boom-lowering, control signal pressure from the pressure of the pilot pump 2.