To provide such a structure that a variable speed operating shaft is protruded outside a transmission case to be slided and rotationally moved with a variable speed lever located relatively far therefrom, at a low cost and with a good feeling for shift operation.

A shift mechanism (a), in which one of two shift forks 50, 51 is selected by the slide of an operating shaft 52 and the selected shift fork 50 or 51 is shifted with the rotational movement of the operating shaft 52, and a PTO variable speed lever 55 supported in a freely crosswise oscillatable manner are provided. The operating shaft 52 and the PTO variable speed lever 55 are connected together in association by engaging a slit 54a formed in one of them with a pin 58 provided on the other, so that the operating shaft 52 can be slided and rotationally moved with the crosswise oscillation of the PTO variable speed lever 55.