PURPOSE: To enable the captioned device to be compact by supporting one end of a wheel driving axis whose the other end is supported to be capable of rotating freely to a crank case cover within a hollow drum portion on a crank axis line, in a device in which the axle of a driving wheel is provided on the crank axis line.

CONSTITUTION: A driving device 6 of an automatic tricycle has a crank axis 14 which is rotated by vertical motion of a piston 12 of an engine. And its left axis portion 14b is provided by a bearing 17 to a left crank case 16 and a generator 19 is provided at the end portion which extends outside of the case. Further, a right axis portion 14c is provided by a bearing 30 to a right crank case 29, and a hollow portion 31 is formed along an axis line. And one end of a driving axis 34 in which a right rear wheel 7 as a driving wheel is provided is provided by a bearing 35 to a right crank case cover 33. And the other end of the axis 34 is supported within said hollow portion 31 through a needle bearing 36 capable of rotating freely. Thereby space between a driving wheel 7 and the engine can be reduced.