To produce a sufficient output effectively independently of a rotational speed at a differential device side.

An electric motor drive mechanism 23 to drive a rear differential device 31 with the output of an electric motor 5 is comprised of a first speed reducing part 43 to transmit the output of the electric motor 5 to the rear differential device 31 side by reducing the speed with a predetermined reduction ratio, a second speed reducing part 45 to transmit the output of the electric motor 5 to the rear differential device 31 at a relatively low reduction ratio, and first and second intermission means 75, 77 to selectively intermit the output transmission by the first and second speed reducing parts 43, 45.