To prevent the trouble of lowering to an idling state of an engine though a boom cylinder is brought to the state of shortening and driving at the time of the lowering of a boom.

A throttle valve 9s is installed in a supply-side oil passage in a control valve 9 for a boom cylinder 5c, a flow control mechanism 13 adjusting a capacity variable type hydraulic pump 8 is mounted, and load sensing maintaining the differential pressure of a first oil passage 14t on the pressure-oil supply lower side of the throttle valve 9s and the second oil passage 15 of a pressure-oil supply port 9p for the control valve 9 at a specified value and auto-idling setting an engine in idling when the boom cylinder 5c is stopped and setting the number of revolution of the engine at a set value according to an accelerator lever at the time of operation are provided. A pressure sensor 23 controlling the operation and nonoperation of auto-idling is constituted so as to detect the pressure of the first oil passage 14t, and has a timer means 27 linking the pressure sensor 23 and an accelerator control means D so that the engine is transferred to the state of idling after a fixed time passes after the pressure sensor 23 does not detect pressure.