PURPOSE: To utilize a plurality of hydraulic pumps effectively according to the increase/decrease of the oil quantity necessary for an actuator.

CONSTITUTION: The discharge oil passages 4-1, 4-2, and 4-3 of a plurality of hydraulic pumps 1-1, 1-2, and 1-3 are equipped with the respective relief valves 3-1, 3-2, and 3-3, and converge to a main discharged oil passage 7, and are connected with an actuator 9. A sequence valve secondary side oil passage 16 is connected through a sequence valve 15, and a sequence valve primary side oil passage 18 is branched to the main discharge oil passage part on the upstream side from the sequence valve, and the set pressure of the sequence valve is set lower than that of the relief valve 3-1. A pilot oil passage 11-1 is connected with the upstream side of a throttle 20-1 for the first hydraulic pump, and the pilot oil passages of the rest pump group are connected in free selection by a selector valve to the upstream side of the corresponding throttle and the outlet side of a selector valve for connecting and disconnecting each pilot pressure, and a means for detecting the sequence valve secondary side pressure is installed.