PURPOSE: To prevent deterioration in balance between left and right, by providing a checking mechanism for preventing transverse movement of a working device in interlocking with steering operation of front wheels over the set angle.

CONSTITUTION: A quadric link mechanism 6 is capable of lifting, lowering and riving by the first single acting hydraulic cylinders 7 and supported rockingly left and right about the vertical shaft center (P1) at the back of a machine body. The second double acting hydraulic cylinder 8 for rocking and driving the quadric link mechanism 6 left and right is installed. A ring member 20 is fixed to a linking rod 18 and an U-shaped member 21 is simultaneously fixed to the side of the machine body so as to sandwich the ring member 20. The movement of the linking rod 18 is stopped by the U-shaped member 21 to stop the expansion and contraction of the second hydraulic cylinder 8, whereas a pitman arm 10 is capable of further steering operation within the stroke of a spring 15.