PURPOSE: To raise the efficiency of excavation by a method in which in an operation region where the necessary operation amount of a boom is smaller than an arm, the arm is guidingly driven and the boom is driven on follow-up basis, and in the region where the necessary operation amount of the arm is smaller, the order of the operation is inversed.

CONSTITUTION: Since a predetermined moving locus G' is drawn at the pivotal supporting point C of a bucket, rotation angles α, β, and γ are detected by a detector group 8 and put, together with an excavation slope angle θ from an operation board 7, in an arithmetic unit 9. In the unit 9, control mode is judged, and in an operation region (a), main control signal is sent out to an arm cylinder 5 of many necessary operation amounts to guidingly drive it through an amplifier 10, an actuator 14, and an oil-pressure switcher 13. Also, sub-control signal is sent out to the boom cylinder 6 to guidingly drive it through the amplifier 10, an actuator 12, and an oil-pressure switcher 11. Then, in the region (b), inversed signals to the foregoing signals are sent out from the arithmetic unit 9. Thus, the follow-up property can be raised.