PURPOSE: To perform transmission control of backward running and to perform easy and safe backward running, by a method wherein a control lever is engaged with a control pedal so that a control pedal, which is held in a neutral position during non-step on, can be controlled only in a forward running direction.

CONSTITUTION: By stepping on a control pedal 12 in the direction of an arrow mark, an arm 31, a rod 16, a control arm 15, and a transmission control shaft 8 are actuated, and an output shaft 9 is rotated in a forward running direction at a speed responding to a step-on angle of the control pedal 12. Meanwhile, by stepping on the control pedal 12 in the direction of an arrow mark from a neutral position, the output shaft 9 is rotated in a backward running direction at a speed responding to its step-on angle. By rotating a control lever 21 clockwisely during non-step on of the control pedal 12, an engaging piece 28 pushes a free arm 26 to actuate a rod 27, the control arm 15, and the transmission control shaft 8, and the output shaft 9 is rotated in a forward running direction at a speed responding to the control angle of the control lever 21.