PURPOSE: To prevent a shoe of a crawler from being worn and broken due to a difference in running speed between crawlers, by providing synchronization control coupling oil motors for synchronizing the drives of oil motors for driving the crawlers.

CONSTITUTION: Crawler running parts C1, C2, C3, C4 on both sides of a weak ground processing machine are driven by drive oil motors 19, 20 having the same performances. Synchronization control coupling oil motors 12, 13, 14, 15 having the same performances and equal volumetric efficiencies per revolution are disposed between pressure supply sections P1, P2 and the above-mentioned drive oil motors 19, 20. With this arrangement, it is possible to prevent a shoe of a crawler from being worn or broken due to a difference in running speed between crawlers, and to eliminate unreasonable stress concentration to a crawler fixing part even on an off-road ground.