PURPOSE: To prevent the occurrence of troubles caused by maloperations by providing a detector for raising positions of a working tool on the tip of a boom, a detector for excess burden to the working tool above a given burden, and a shutter valve to close a floating circuit during the detecting period of both said detectors.

CONSTITUTION: When a control valve 10 is out of floating, a detector 21 is operated to release a shutter valve 17, and by switching control valves 10 and 12, a boom cylinder 4 and a bucket cylinder 6 are allowed to extend or contract freely. When soil is fully loaded on the bucket 5 and the cylinder 4 is extened, a load detector 18 is operated by discharge pressure of the oil-pressure pump 9 if the burden of the bucket 5 exceeds a given one. When the bucket 5 goes up to a given height, a position detector 20 is operated, a valve drive circuit 19 is actuated, and the shutter valve 17 is excited to interrupt between the control valve 10 and the cylinder 4. The falling down of the bucket 5 by maloperations can thus be prevented.