With the control device, when the traveling state of the vehicle 1A is a specified traveling state in which greater change than a reference value of the torque upon the output portion 11 due to input to the drive wheels Dw from the road surface over which the vehicle is traveling has occurred a plural number of times within a predetermined time interval, then when the selectable one-way clutch 12 is in the locked mode it is changed over to the free mode, and when the selectable one-way clutch 12 is in the free mode it is kept in the free mode.