To effectively perform disassembly and reassembly while preventing a screw from damaging and loosening by restraining the influence of mud and miscellaneous things to screws to mount a mounting bracket pivotally supporting a free rotating wheel as much as possible.

A mounting bracket 19 pivotally supporting free rotating wheels 18 is detachably installed to the lower face 14a of a truck frame 14 through the screw 23 advancing upward from a part beneath the lower face 14a, and a guide body 20 restricting an elastic body crawler 17 from coming off is extended in the crawler winding direction at a position directly below the head of the screw 23.