PURPOSE: To insure an oil amount of a breaker, by a method wherein the pressure oil of a second variable volume hydraulic pump for driving an actuating except the breaker is connected to the regulator of a first variable volume hydraulic pump for the breaker through an on-off valve.

CONSTITUTION: An ON-OFF valve E, which is closed during driving of a breaker, is mounted in a regulator circuit 62 interconnecting a variable volume type hydraulic pump 3 and a first regulator A, and an oil pressure breaker 10 is connected to a first variable volume type hydraulic pump 2 through a switching valve group 4. Meanwhile, an arm cylinder 9 is connected to a second variable volume type hydraulic pump 3 through a switching valve group 5. A discharge pipe of the first hydraulic pump 2 is communicated to the regulator A through a line a1 and to a regulator B for the second hydraulic pump 3 through a line a2, and the discharge pipe of the second hydraulic pump 3 is communicated to the regulator B through a line b1. This permits ensuring of an oil amount of a breaker during composite operation.