An automatic debiting system and method suitable for free lane traveling. Debiting antennas disposed on a first gantry are used to communicate with an in-vehicle unit (IU) mounted on the vehicle for debiting. The passages of the vehicles are detected by the loop coils or line scanners, and the license plates, etc., of the vehicles are photographed by the enforcement cameras. Debiting confirmation antennas on second gantry are used to communicate with the IU for the confirmation of debiting. When the normal debiting is confirmed, a local controller informs a system central controller of the fact, whereas when abnormal debiting is confirmed, images of the license plate, etc., of the illegal vehicle are transmitted to the system central controller as illegal vehicle images. The debiting is thus possible at the time of free lane traveling.