PURPOSE: To improve the work operationability by controlling a pressure reducing valve interposed in a pipeline for communicating a pilot combining valve and a pilot valve of a remote control valve for a boom, and enlarging a ratio of a boom hoisting height against an angle of turning.

CONSTITUTION: A solenoid proportional pressure reducing valve 50 is interposed in a circuit 32b for communicating a pilot pressure receiving part 65 of a pilot combining valve 64, which is provided in a pipeline for supplying the pressure oil from a second pump to a turning motor 2, and a pilot valve 20 of a remote control valve 19 for a boom. When the pilot pressure is not led out from the pressure reducing valve 50, the combination supply is not performed to a boom cylinder 1. Next, a combination supply side passage of the combination valve 64 to the boom cylinder 1 is opened by the pressure reduction setting operation of the pressure reducing valve 50, and simultaneously, an open degree of the passage to the tuning motor 2 is reduced. A ratio of a boom hoisting height against an angle of tuning is thereby increased to improve the operationability.