To make the operating direction of a speed change lever, raising/ lowering a change gear ratio, coincide with the direction of a driver's seat by putting a transmission in speed change action interlockingly with the longitudinal operation of the speed change lever and also in forward/backward switching action interlockingly with the lateral operation of the speed change lever through a forward/backward switching part.

A lever guide hole 18 of nearly crank shape extended in a longitudinal direction is formed at the side of a driver's seat, and a speed change lever 9 erected tiltably in longitudinal and lateral directions is inserted through the lever guide hole 18. A forward/backward switching part 20 is formed at the base end part of the speed change lever 9. Through this forward/ backward switching part 20, a transmission can be put in speed change action interlockingly with the longitudinal operation of the speed change lever 9 and also in forward/backward switching action interlockingly with the lateral operation of the speed change lever 9. The operating direction of the speed change lever 9 raising/lowering a change gear ratio can thereby coincide with the direction of the driver's seat.