PURPOSE: To cope with a large load such as escaping from a wet field while enabling durability to be improved as a transmission gear total unit by selectively transmitting to a driving shaft in a running system speed change power from a friction continuously variable transmission mechanism and non-speed change power not through this mechanism.

CONSTITUTION: A transmission device is formed by an input shaft 1 receiving power from an engine, friction continuously variable transmission mechanism A formed around the shaft 1, cutoff means B of power, synchronous clutch C, center differential gear mechanism D distributively transmitting power from the transmission mechanism A to rear and front wheel drive shafts 2a and 2b, transmission shaft 4 of PTO system, power on-off means F, etc. And an on-off shifter 25 of the on-off means F is placed in a transmission cutoff condition, when a shifter 11 of the cutoff means B is shift to either right or left, speed change power can be transmitted to the running drive shafts 2a, 2b. Next the shifter 11 is placed in the neutral, further when the on-off shifter 25 is placed in a transmission condition, non-speed change power not through the friction continuously variable transmission mechanism A can be transmitted to the running drive shafts 2a, 2b.