There is provided a traveling direction vector reliability determination method in which reliability of a traveling direction vector of another vehicle is calculated so as to increase reliability of a collision prediction. The traveling direction vector reliability determination method determines the reliability of the traveling direction vector when the traveling direction vector is calculated based on position coordinate points of a target, which are calculated by a radar device. The method includes a traveling direction vector calculation step of calculating, based on a movement history of the position coordinate points, the traveling direction vector of the target; and a reliability calculation step of calculating, in a case where the position coordinate points include normally recognized coordinate points and estimated coordinate points, the reliability of the traveling direction vector, based on at least one of information about the normally recognized coordinate points and information about the estimated coordinate points.