The forward/reverse travel changeover mechanism includes a projection formed by causing an output power shaft for outputting the power from a main transmission to project to the outside of a case, a carrier rotatably supported around the projection relative to a sun gear provided rotatably integrally with the projection, a planetary gear, an internal gear held in meshing engagement with the planetary gear, an output sprocket rotatably supported integrally with the internal gear and rotatable around the projection adjacent the main transmission with respect to the sun gear, and a shifter for selectively changing over the position of the carrier and the internal gear between a forward travel position at which the carrier and the internal gear are connected against relative rotation and a reverse travel position for preventing rotation of the carrier around the projection, and is disposed outside the main transmission case.