To simplify a structure for lifting up and down a crawler, especially the structure of a truck frame, prevent a number of muddy balls from being generated, and improve travellability in an ill-drained paddy field.

The upper parts of a plurality of equalizer frames 4a, 4b which include a plurality of ground-contact rotating wheels 3 are pivoted respectively through supporting shafts at a truck frame 2 where an idler 1 is suspended at the rear part. The top of a parallel links 7a, 7b which are pivoted at a supporting frame 6 positioned at the lower part of an machine frame are pivoted at both the spindles respectively. Both the links 7a, 7b are transmitted and communicated with a hydraulic cylinder 8 upward so that they may rotate, and a crawler is fitted at a driving wheel 9 upward on the front side, a tension wheel 10 upward in the middle, an idler 1 on the rear side, and the respective rotating wheels 3. The tension wheel 10 upward in the middle is suspended at the supporting frame 6.