A lane boundary detector includes an edge point extracting unit that extracts a plurality of edge points of a lane sign from the input image, and an edge line extracting unit that extracts a plurality of edge lines of the lane sign from the edge points to formulate a position and a direction of each of the edge lines as a straight line on the road surface, using a parameter set consisting of at least two variables. The detector further includes a branch point detecting unit that determines whether a crossing point of two of the edge lines can be a branch point of the lane, based on whether a characteristic difference vector defined with the variables with respect to the two edge lines falls within a predetermined appropriate range, and a lane boundary determining unit that determines the left and the right boundary positions of the lane.