PURPOSE: To dispense with space for slide operation of cam member and the like so as to make compact a changeover structure and a safety structure by internally fitting the changeover structure and safety structure by which a front wheel transmision is not operated to an acceleration condition side, in a fitting part between a control rod and a boss part of a shift fork.

CONSTITUTION: A projection 20a is provided at a control rod 20 while a first and a second recesses 34a, 34b are provided at a boss part on the side of a shift fork 19. The boss part 34 is rotation operated by a first operation means 37 and thus the projection 20a is inserted into the first recess 34a. When front wheels are steered at a specified angle or more from a linear line position, under the above condition, due to the action of a cam mechanism, the rod 20 is slid to an acceleration condition side. In this case, since the projection 20a is inserted in the recess 34a, due to the engagement of the projection 20a with the recess 34a, the shift fork 19 is operated by slicling leftward together with the rod 20 and a front wheel transmission is changed over to the acceleration condition.