PURPOSE: To reduce cost by omitting a handle switch for detecting the locking release operation of a door handle.

CONSTITUTION: In a sliding-door closing device, in which a control unit 40 conducting idle-swing preventive drive reversely rotating and driving a motor 12 at a time when the motor 12 is forward rotated and driven automatically under the state of the half locking of a door locking device and the door locking device is full-locked and the locking release operation of a door handle is carried out during the forward rotation and driving of the motor 12 is mounted, a half locking switch 7 is used as the detecting means of the locking release operation of the door handle. The control unit 40 starts the measurement of a timer by the first closing of the half locking switch 7, and performs the idle swing preventive drive of the motor 12 when the closing of the half locking switch 7 is released during the measurement of the time of the timer.