A vehicle side door opening/closing device includes a motor, a gear case to which the motor is fixed and which is fixed to a side door, a speed reducer built in the gear case, an output gear provided to the speed reducer, a rack of which an end portion is pivotably supported to a vehicle body and which is provided with a rack gear, and an engagement holding unit connected with the gear case and movably supporting the rack while constantly keeping an engagement allowance between the rack and the output gear. The side door is moved in an opening direction by pushing the rack and in a closing direction by pulling the rack respectively following the rotation of the output gear. The engagement holding unit rotates around a rotation center of the output gear by a swing of the rack following the opening/closing operation of the side door.