PURPOSE: To improve the extent of safety in preventing any sudden turning at time of high speed travel in a state of connecting a farm working machine by installing a safety device which checks operation of a side clutch in linkage with a changeover to the high speed position of a traveling gear shifter.

CONSTITUTION: An outer wire 11b of a release wire is supported on a transmission case M via a receiving seat 13 and a guide member 14 free of vertical motion. This receiving seat 13 is energized to a side where an inner wire 11a is loosened by a spring 15. In addition, a cam member 16 being free of rocking motion around a horizontal shaft 16a is made contact with the receiving seat 13. In this cam member 16, there is provided a recess B which releases a moving regulation for the receiving seat 13 when a main gearshift lever 6 is operated to at least high speed positions F7, F8 of a shift guide plate 12. With this constitution, the receiving seat 13 is moved to a loose side of the inner wire 11a, thereby absorbing operating force of a side clutch lever with this loose part, and such a checking state that operation of a side clutch 9 is impossible is thus secured.