PURPOSE: To facilitate steering change-over of front and rear wheels by enabling the change-over immediately after starting by use of main and auxiliary circuits.

CONSTITUTION: There are provided a cylinder 4 for changing over the running direction steering conditions of front and rear wheels incorporated in a forward/ backward running change-over device; an automatic controlling gear 1 which gives a control instruction to a running motor 5 and the cylinder 4 which serve as loads; an auxiliary circuit 2 which feeds the instruction from the gear 1 to both loads only in initial several seconds; and a main circuit 3 which checks the operation of the cylinder 4 by the function of the circuit 2. With the circuit started operation, the instruction from the gear 1 is fed to both loads even after the stoppage of the circuit 2; the steering change-over corresponding to the running of front and rear wheels by the cylinder can be conducted after the motor 5 has set in acceleration stage. Thus the steering change-over can be checked correctly, and safely is improved.