PURPOSE: To prevent the collision of a bucket on an operator's cabin by a method in which a solenoid switch valve is provided between the bucket cylinder and its control valve and when each cylinder is extended to a fixed amount, the solenoid switch valve is excited to supply pressure oil to the boom cylinder and the arm cylinder.

CONSTITUTION: In the demagnetized state, the bucket cylinder 1 and the control valve 6 are led to the pipeline of the bucket cylinder 1, and in the excited state, pressure oil from the control valve 6 is returned to the valve 6 and supplied to the compression side of the boom cylinder 2 and also to the extension side of the arm cylinder 3 by means of a solenoid switch valve 4. Switches 8, 9, and 10 to be closed when the boom cylinder 2, the arm cylinder 3, and the bucket cylinder 1 are extended in a fixed amount are connected in series to an electric circuit connecting the exciting portion 4d of the solenoid valve 4 with a power source 7. The collision of the bucket on an operator's cabin is prevented and the operational range of construction machine can thus be widened.