To prevent rapid turn and to prevent wasteful long turning time by restricting the action of a front wheel speed increasing means according to the changing condition of a tread when a vehicle is turned, or changing the degree of speed increase of the front wheel.

A controller 29 judges whether or not the length of the tread is not less than the width to be imagined unsuitable for the front wheel turning at increased speed. If the result of judgment is true, the front wheel speed increase line which is set so that the degree of the front wheel speed increase is reduced more as the tread becomes narrower, is read in a RAM, and the degree of the speed increase is regulated based thereon. If the result of judgment is false, a solenoid 65S of a control valve is energized to restrict the action of the front wheel speed increase. Even in a case where both a tread changing device and a front wheel speed increase device are provided, the raped turn of the vehicle body through the excessive speed increase of the front wheel is eliminated when the tread of the vehicle is small. Because the degree of the front wheel speed increase is increased more as the tread is large, the rapid turn becomes possible, and the work becomes efficient.