To improve the operation in the turning, by controlling the rotating speed of the travel wheels in such manner that the high speed turn control is performed in an initial period of the turning operation, and the low speed turning control is temporarily performed in the end half of the turning.

In a case when a handle is turned in the left and right directions by the control for the turning, a machine body changes a direction corresponding to a turning angle, and further an output to a motor M2 is transmitted to switch a switching valve, when the turning angle is changed over 90 degree. Then a side brake SB is started to operate simultaneously with 'OFF' of a side clutch at the turning angle of about 180 degree, and further the side clutch is switched 'ON' when the turning angle is almost 400 degree by the returning operation of the handle, thereby the side brake SB is operated. An output signal is transmitted to a motor M1 by a signal from a turning angle detector at the almost 400 degree of turning angle before or after the operation of the side brake, and a V belt 39 is pressed by a tension roller 40, thereby the transmission to an input axis 37 from a split pulley 36, is reduced, and the machine body is slowly turned.