PURPOSE: To obtain a pollution-free self-traveling vehicle by removably connecting a hydraulic power source to a self-traveling device via couplers, rotating a flywheel at a high speed by means of pressure oil from the hydraulic power source to accumulate energy and, after that, rotating a driving wheel with the flywheel as the driving source.

CONSTITUTION: At the time of driving a self-traveling device, first, a hydraulic power source 22 driven by a prime mover 21 is connected to the self-traveling device via couplers 24 and a pressure oil from the hydraulic power source 22 is fed to a piston pump motor 2 under a condition of the maximum inclination angel of its swash plate 81 via a selector valve 23 to rotate a flywheel 1. When the rotating speed of the flywheel 1 becomes high speed, the selector valve 23 is switched over and the couplers 24 are disconnected. At this time, the swash plate 81 is made a right angle to a pump shaft 84 to eliminate energy consumption in in the piston pump motor 2. By switching over the next selector valves 3, 13, pressure oil discharged out of a sub-pump 12 which is driven by the flywheel 1 is fed to a piston motor 4 in normal/reverse directions thereby rotating driving wheels 6 in normal/reverse directions to advance/back the self-traveling device.