PURPOSE: To make compact the structure of a multistage drive unit by installing an internal control means in a transmission device for changing over an epicyclic gearing device according to the revolving speed, and accommodating all rotary parts in a casing.

CONSTITUTION: When the engine speed exceeds a changeover speed, a centrifugal weight 28 moves outward in the radial direction from the bore of a conical roller 7b. A pressing disc 29 is pushed left together with a tappet 32, and an oil pressure supply passage 2 generates communication with a return passage 33 leading to a cylinder chamber 34, and a piston 35 is pressed till colliding with the inner side member 24 of a brake, and the frictional engagement of brake discs 22 and 23 is released. The outer member 19 is disengaged to rotate together with a planet gear 10, and torque transmitting from a drive shaft 6 to a driven shaft 12 is made through an intermediate shaft 7 and a free wheel 14. Thus the drive unit can be made compact, and changingover of the transmission device can automatically be performed simply and quickly.