An electronic control device, comprising: a lane boundary detection unit configured to detect at least one of a first lane boundary and a second lane boundary based on image information on an area in front of a vehicle, the first lane boundary and the second lane boundary being positioned on a road surface on which the vehicle travels, and the second lane boundary being positioned on the road surface on an opposite side of the first lane boundary across the vehicle in a vehicle width direction; and a lane boundary marking line search unit configured to, when one or the other of the first and second lane boundaries cannot be identified, search for a lane boundary marking line on the road surface on a side horizontally opposite a position of a first detected lane boundary, wherein the lane boundary marking line search unit is configured to project search lines on the road surface on the side horizontally opposite to the first detected lane boundary, the search lines having a curvature and a lateral position representing a shape of the road on which the vehicle travels where the first lane boundary is detected, the search lines being spaced at intervals from each other across the vehicle width direction in the lane, characterized in that the lane boundary marking line search unit is configured to: acquire search line brightness information based of the image information, the search line brightness information being brightness information on each of the search lines, select a search line having a maximum value and a local maximum value of average brightness among the search lines as a lane boundary marking line candidate line based on the search line brightness information, and when a search line having the maximum value and the local maximum value of average brightness is not determined, disregard all of the search lines as not constituting lane boundary marking line candidate lines. a lane boundary detection unit configured to detect at least one of a first lane boundary and a second lane boundary based on image information on an area in front of a vehicle, the first lane boundary and the second lane boundary being positioned on a road surface on which the vehicle travels, and the second lane boundary being positioned on the road surface on an opposite side of the first lane boundary across the vehicle in a vehicle width direction; and a lane boundary marking line search unit configured to, when one or the other of the first and second lane boundaries cannot be identified, search for a lane boundary marking line on the road surface on a side horizontally opposite a position of a first detected lane boundary, wherein the lane boundary marking line search unit is configured to project search lines on the road surface on the side horizontally opposite to the first detected lane boundary, the search lines having a curvature and a lateral position representing a shape of the road on which the vehicle travels where the first lane boundary is detected, the search lines being spaced at intervals from each other across the vehicle width direction in the lane, characterized in that the lane boundary marking line search unit is configured to: acquire search line brightness information based of the image information, the search line brightness information being brightness information on each of the search lines, select a search line having a maximum value and a local maximum value of average brightness among the search lines as a lane boundary marking line candidate line based on the search line brightness information, and when a search line having the maximum value and the local maximum value of average brightness is not determined, disregard all of the search lines as not constituting lane boundary marking line candidate lines. select a search line having a maximum value and a local maximum value of average brightness among the search lines as a lane boundary marking line candidate line based on the search line brightness information, and when a search line having the maximum value and the local maximum value of average brightness is not determined, disregard all of the search lines as not constituting lane boundary marking line candidate lines.