PURPOSE: To vary a flow rate, by a method wherein a switching valve is located on plural discharge amount control circuits for controlling the sweeping volume varying mechanism of a hydraulic pump, and the control circuits are energized and shut off according to a working condition.

CONSTITUTION: When a normal excavating work is conducted, with switcheds 50 and 51 closed, electromagnetic valves 46 and 47 are switched to a position on the left side, and oblique plates 14a and 15a of hydraulic pumps 14 and 15 are brought to a minimum inclination. This causes a boom cylinder 9 and an arm cylinder 10 to be driven by hydraulic pumps 16 and 17, and running motors 6 and 7 to be respectively driven only by the single pump. Meanwhile, when an attachment having a low volume is driven by an empty switching valve 19, if only the switch 50 is closed and the switching valve 46 is shifted to a position on the right side, the attachment is driven only by the single hydraulic pump, and breakage due to an excessive flow rate is prevented from occurring. In this case, the cylinders 9 and 10 are driven at a high speed with the aid of the three pumps.