PURPOSE: To prevent an erroneous operation by temporarily disengaging a clutch via an interlocking mechanism at the time of rotating a speed change lever while, on the other hand, disengaging the clutch via a means with a larger operating resistance at the time of rotating same from the neutral to the opposite direction.

CONSTITUTION: In the captioned device, a speed change power transmission mechanism is changed in its speed in multiple stages gradually from the neutral to advancing high speed side by the rotation in one side direction of a speed-change lever 1, while being switched over to the backing side by rotating same from the neutral position in the opposite direction. In this case, an interlocking mechanism 4 for switching over a clutch 2 provided for connecting/disconnecting power transmission to the speed change power transmission mechanism, to a disengaging side by the movement to one side direction of an engaging roller 3 which is engaged accompanying the rotation of the speed change lever 1 is provided on the clutch 2 side. On the other hand, a clutch operating board 7 having a plurality of projections 5 engaged with the engaging roller 3 and a projection 6 with a shape of more increasing the operating resistance to the speed change lever 1 than by the projections 5 is provided on the speed change lever 1 side.