A division line depicted on a road is detected. A beam-formed electromagnetic wave is repetitively at intervals transmitted toward the road viewed from a vehicle to scan in the vehicle width direction. Each beam-formed electromagnetic wave is radiated to a radiation area on the road, and the radiation areas made by transmitting the beam-formed electromagnetic wave a plurality of times virtually produces a scan area on the road. Every beam-formed electromagnetic wave, distance data indicative of a distance between a division line on the road and the vehicle is measured based on information about a reflected electromagnetic wave. The distance data measured is received to detect the division line based on characteristics of changes in a sequence of the distance data produced by mapping the received distance data in a scanning order of the beam-formed electromagnetic wave.