PURPOSE: To reduce deformation of a hollow shaft and support a transmission gear without fail by making an additional housing compact by allowing a differential gear machanism to closely approach the transmission gear, and further supporting the middle of the hollow shaft.

CONSTITUTION: A central differential device 10 differentially distributes drive force to the front wheels 33, 34 side and the rear wheels 74, 75 side, and absorbs a differential effect at turning of the car or a diameter difference between wheel mounted to the car to enable the car to be subject to four-wheel drive traveling at all times. Then, when the internal teeth part 39 of the coupling sleeve 36 of a lock-up mechanism 42 is engaged with the external teeth part 41 of a clutch 40, and a second hollow shaft 16 and a differential case 19 are coupled together and made in a locked state via a first hollow shaft 15, the central differential device 10 stops the differential distribution, and the first and second hollow shafts 15, 16 are integrally rotated, and the front and rear wheels 33, 34, 74, 75 are also integrally rotated, enabling the car body to escape from hard driving condition etc. Further, a boss part 45 constitutes a middle support part.