In a collision avoidance ECU (1), a travel trajectory determined based on an estimated curve radius of a vehicle is estimated by a basic-travel-trajectory estimation section (12), the distance between the vehicle and a white line is obtained by a changed-travel-trajectory estimation section (15) based on a relative positional relationship between the vehicle and the white line, and, with the obtained distance maintained, a route along the white line is estimated as a changed travel trajectory. When automatic steering control and deviation alarm control are not being performed, a collision determination section (17) performs collision determination based on the basic travel trajectory estimated by the basic-travel-trajectory estimation section (12). When the automatic steering control and deviation alarm control are being performed, the travel trajectory is changed to the changed travel trajectory estimated by the changed-travel-trajectory estimation section (15).