A sliding-door drive device includes: a motor; a speed reducing mechanism reducing rotation of the motor; opening and closing cables having respective one ends secured to a sliding door of a vehicle; an opening drum, having the opening cable wound therearound, to be rotated by the motor through the speed reducing mechanism; and a closing drum, having the closing cable wound therearound, to be rotated by the motor through the speed reducing mechanism. Further, rotation axes of the motor, the opening drum, and the closing drum are parallel, and the opening drum and the closing drum are arranged parallel in radial directions thereof.