To keep three other wheels in driving even if one of the wheels goes in idling using a simple configuration without providing any flow control valve or the like for restricting the differential motion.

A hydraulic motor 22a for driving the left front wheel is connected in series to a hydraulic motor 23a for driving the left rear wheel through a pipe line 26a, while a hydraulic motor 22b for driving the right front wheel is connected in series to a hydraulic motor 23b for driving the right rear wheel through a pipe line 26b. The series oil circuit of the left-hand motors 22a and 23a is connected parallel with the series oil circuit of the right-hand motors 22b and 23b. When the vehicle is advancing, a working oil is supplied and exhausted by a working oil control valve 25 so that the motors 22b and 23b for the rear wheels 13L and 13R to serve steering whose revolving speeds become high relatively are positioned upstream and the other motors 22a and 23a on the stationary side are positioned downstream.