A process is repeated in which N state vectors each including, as elements, the position of a camera with respect to a vehicle, the posture of the camera, the width of a lane where the vehicle is driving, and the orientation of the vehicle with respect to the lane are changed according to a predetermined dynamics model, a reference state vector representing the positions of edges of lane marks included in an image captured by the camera is generated when each of the changed state vectors is true, considering the positions of edges of lane marks in the image as an observation vector, the degree of match between the reference state vector and the observation vector is calculated as the weight of the state vector, a weighted average vector of the state vectors is calculated, and N state vectors are resampled on the basis of the calculated weights.