Sliding-door drive unit for a vehicle

A sliding-door drive unit for a vehicle is capable of transferring or blocking drive power required to pull or push a cable to open or close a sliding door. The sliding-door drive unit includes a worm wheel rotatable by a drive motor, a stationary disc coupled to an output shaft, and a movable disc rotatably installed between the worm wheel and the stationary disc along an axial direction of the output shaft. The movable disc is engaged with the worm wheel and is selectively engaged with the stationary disc.