PURPOSE: To safely perform multi-stage speed change travelling by stopping operation by allowing a first limiting arm to collide with a first stop part while a second limiting arm to collide with a second stop part.

CONSTITUTION: First stop parts 10h, 10l, 11h, and 11l are provided on first speed change operation boss parts 10, 11 while front and rear second stop parts 14h, 14l are provided on a second speed change operation boss part 14 of a second speed change mechanism. A push movement piece 20 is provided on the first change lever 6 of the first speed change mechanism in which a direction of an operation part 6c can be changed following front/rear change operation of the direction of a steering handle 2. The push movement piece 20 is constructed interlocking with first stop parts located in front and back of the first speed change operation boss part so as to enter therebetween. The first limiting arm 15 and the second limiting arm 16 are in a backward high speed state of the first speed change mechanism, and high speed state operation of the second speed change mechanism is stopped with interference operation among the respective stop parts.