PURPOSE: To obtain interchangeability between two and four-wheel drives by providing a transmission system from a front wheel reduction gear through a transfer unit to the rear wheel side in a two-wheel driven car with an automatic speed change gear.

CONSTITUTION: During a four-wheel drive, a transfer drive shaft 31 is coupled with the drive pinion 17 of the front wheel reduction gear 3 of a two-wheel driven car to take out the power to the rear wheel. This shaft 31 is extended backward through a gap between an automatic speed change gear 5 and the valve block 20 of an oil hydraulic control unit. In addition, a plate-like partition body 32 concurrently serving as the sealing of an oil pump 21 is united with the rear end of a speed change gear case 6, and an extension case 33 is connected through this partition body 32. Then, a transfer unit 34 containing a rear drive shaft 35 is fitted into this case 33 so that the shaft 35 can be coupled or uncoupled with a driven gear 37 through the oil hydraulic clutch 38 for switching between two and four-wheel drives.