The present invention provides zigzag state detecting section which is comprised of white line determining section for determining parts of the road surface which are similar to white lines, white line position detecting section for detecting positions, approximating section for approximating the positions, left-hand zigzag amount determining means for calculating a zigzag amount of a left-hand boundary, right-hand zigzag amount determining section for calculating a zigzag amount of a right-hand boundary, and calculating section for calculating an average zigzag amount of the vehicle on the basis of results of the calculations by the left-hand zigzag amount calculating section and the right-hand zigzag amount calculating section.