An appearance of a vehicle is improved by reducing an opening formed in an interior trim for vehicle. An actuator unit is mounted in a pillar of the vehicle, and a linking rod is linked to a rack bar, which linearly reciprocates along an axial direction provided at this actuator unit. This linking rod protrudes from the pillar through the opening formed in an interior trim, and the other end thereof is linked to a back door. Moreover, this actuator unit is disposed so that a moving direction of the rack bar inclines on a vehicle-rear side by an opening/closing inclination angle &agr; with respect to a direction perpendicular to the vehicle and inclines inward in a vehicle-width direction by a width inclination angle.