Lane boundary detector

A boundary detector includes a candidate line detecting unit (23) that detects a candidate line of a lane boundary position drawn on the road surface based on image information from a camera (11). The detector also includes a lane boundary position selecting unit (24) that selects the lane boundary position based on the candidate line; an additional lane boundary position selecting unit (26) that selects a lane boundary position of an additional lane that is added to a main lane based on the candidate line; and an additional processing unit (26) that stops adopting the lane boundary position for a predetermined time period when the additional lane boundary position selecting unit (26) selects a lane boundary position of the additional lane.