To enhance steerability by making the control angle of a steering wheel adjustable to match both the travel of a working vehicle and actual operations.

A potentiometer 9 detects the control angle of a steering wheel 8, and this detected value Vi is input to a controller 11. The controller 11 outputs a command signal V0 to a solenoid valve 7, and a steering valve 4 is operated by pilot pressure guided to the steering valve 4 via the solenoid valve 7, driving steering cylinders 1, 2. Plural kinds of gains for V./Vi are set, and any of the gains can be selected by the on-off control of a changeover switch 10. Therefore, safe travel is ensured at high speed because of a small V0/Vi, whereas during actual operations the V0/Vi is increased to ensure a large steering angle even at a small steering-wheel control angle so as to relieve the driver of fatigue.