Traffic accidents are caused by various causes such as human factors : carelessness of vehicle drivers, etc, vehicle defects : braking system defects, etc and environmental factors. It is required to identify whether the accident is caused by careless of the driver or vehicle defect because of the legal responsibility of the driver. This case is an accident which the vehicle named chairman drove backwards on the road after refueling at a gas station and crashed into the wall of a building 200m away, and the driver claims it was a sudden acceleration accident due to a vehicle defect. Through the trace of the accident vehicle, trace formed on the driver's shoes, vehicle inspection and CCTV analysis, it is determined that the accidents are caused by carelessness of the driver who press the accelerator by mistake. If a similar case occurs in the future, it is thought to it can be used to determine the cause of the accident.