Abstract: A tractor and a trailer are connected together and a camera is installed on the trailer side of the connected vehicles. The camera captures images behind the trailer. A driving assistance system projects the captured images on bird's-eye view coordinates parallel with a road surface to convert the images into bird's-eye view images and obtains on the bird's-eye view coordinates an optical flow of a moving image composed of the captured images. The connection angle between the tractor and the trailer is estimated based on the optical flow and on movement information on the tractor, and further, a predicted movement trajectory of the trailer is obtained from both the connection angle and the movement information on the tractor. The predicted movement trajectory is overlaid on the bird's-eye view images and the resulting image is outputted to a display device.