To improve the travel speed change and steering functions of crawlers, and to achieve other aims such as simplifying travel speed change and steering control, by automatically varying the steering amount of the steering mechanism in proportion to the speed change amount of the speed-change mechanism.

This vehicle is equipped with a hydraulic continuously variable speed change mechanism for straight travel (speed change mechanism) 25 which drives left and right crawlers in the same direction at the same speed, and a hydraulic continuously variable speed change mechanism for turning (steering mechanism) 28 which drives the left and right crawlers in opposite directions at the same speed. A main gear shifting lever 68 which is the controlling portion that links to the speed change mechanism 25, and a steering wheel 19 that is linked to the steering mechanism 28 are interlocked by the speed change and turning interlock mechanisms. The steering amount of the steering mechanism 28 is automatically adjusted according to the calculated output of the controller in proportion to the speed change amount of the speed change mechanism 25. This means that the driver can manipulate speed changes with the same feel as when driving a 4-wheeled vehicle, and the speed change and steering functions of the crawlers are improved, and travel speed changes and steering control are simplified.