A slidable and rotatable first control shaft (61) and a slidable and rotatable second control shaft (67) for respectively shifting main and auxiliary speed change mechanisms (26, 29) project outwardly to one side of a vehicle housing (2) and are connected respectively to a main change lever (74) and an auxiliary change lever (75) which are pivotally supported by a support member (70) fixedly provided at one side of the vehicle housing. Preferably, the main change lever is disposed at a location where an operator on a seat (76) can operate the lever easily whereas the auxiliary change lever is disposed at a location where the operator is hard to operate the lever. The support member is preferably composed of a cover member which surrounds the control shafts, lower portions of the change levers, and connecting members (80, 87) therebetween. An opening (70b) provided to the cover member for assembling the connecting members is closed by a closure member (70c).