PURPOSE: To facilitate adjusting operation and prevent an unintended movement of the captioned device when it is not operated by providing a speed change lever between a driver's seat and one fender while providing an adjusting member for adjusting the interlocking condition between an operating mechanism and a restraining mechanism, on the outside part of a transmission case.

CONSTITUTION: A speed change lever 7 is provided between a driver's seat 5 and a fender 6, while a restraining mechanism 24 which restrains the unintended movement of one, which is not being operated, of two sets of speed change operating parts which are selectively operated by means of the speed change lever 7, and an adjusting member 20 for adjusting the interlocking condition between the operating mechanisms 17, 21 of the speed change operating parts, are provided on the outside of a transmission case 11. Accordingly, an adjusting mechanism 21 placed on the outside of the transmission case 11 can be easily adjusted, and a restraining mechanism 24 can be always accurately operated, to securely carry out a selective operation of two sets of speed change operating parts by means of one speed change lever as well as an unintended movement checking operation for the speed change operating parts.