PURPOSE: To enable the absorbing limit of a volumetric change absorbing mechanism to be altered by effecting the transmission from one side of front and rear wheels to the other side through oil pressure generated in a hydraulic pump constituted on the way of a transmission system to both wheels, while absorbing the volumetric expansion accompanying the temperature rise of working oil in the hydraulic pump by the volumetric change absorbing mechanism.

CONSTITUTION: A volumetric change absorbing mechanism 5 has a pressure receiving spool 52 disposed in a cylinder chamber 51 communicating to the interior of a casing of a hydraulic pump through a pressure guide 50 to be slid by receiving the internal pressure in the casing increased along with the volumetric expansion of working oil so that the volumetric expansion is absorbed by the volumetric increase of the cylinder chamber 51 along with the slide. A plurality of stoppers 54, 54... are disposed on the side of the pressure receiving spool 52 opposite to the pressure receiving surface to limit the slide range of the pressure receiving spool 52 by the spool abutting against the innermost stopper 54. The absorbing limit of the volumetric change absorbing mechanism 5 can be altered by increasing or decreasing the number of the stoppers 54, 54... to alter the position of the innermost stopper 54.