PURPOSE: To faciliate smooth operation by installing a hydraulic cylinder in the hydraulic circuit between a foot pedal and a brake and installing a variable- pressure control valve in an automatic direction controlling circuit and controlling the brake force in the cases of automatic control and pedal operation.

CONSTITUTION: When a foot pedal 18 is stepped-in, the piston of a hydraulic cylinder 29 is pushed down, and an inlet 49 is closed and pressure is applied into a hydraulic piping 31 and a cylinder 30, and turning of an axle 16 is braked by the operation of a brake 25, and the car stops or turns. When the advance direction shifts leftward in automatic direction control, and a grain handling lever 6 is brought into contact, the solenoid 37 at right hand operates, and a solenoid valve 38 is opened, and the pressurized oil from a hydraulic pump 33 is transported from the inlet 49 into the cylinder 29 through a valve 38 and a variable- pressure control valve 41, and further transfers a rod 28 rightward through a piping 31 and the cylinder 30, then the axle 16 at right hand is braked, and the advance direction is corrected. As the hydraulic pressure is regulated to the pressure set by the control valve 41, the brake force of a brake 23 is not excessively increased, so sharp turning is not generated.