PURPOSE: To provide a mechanism which can still travel at ultra low speed even if rotating speed of an engine of a truck is increased by rotating a driving wheel by driving a transfer through a propeller shaft by rotation of a hydraulic motor, and also selecting speed optimal for work as hydraulic driving only at work time.

CONSTITUTION: A hydraulic variable pump 5 and a hydraulic motor 4 are arranged on a support frame of a working truck A, and motive power of an engine is transmitted to the hydraulic variable pump 5 through a propeller shaft 6 from a PTO of a transmission 8, and this is driven. The hydraulic motor 4 is driven by oil pressure generated by the hydraulic variable pump 5, and shaft rotation of the hydraulic motor 4 is transmitted to an input shaft of a transfer 10 through a propeller shaft 7. Next, travel is performed by output of this transfer 10 by rotating a driving wheel by an ordinary machine driving system, and at ordinary traveling time, the transmission PTO and a clutch of the transfer are turned off.