A course estimator for a drive assist system, comprising: a first estimating means (40, S110 to S130) for obtaining first information and estimating a first radius of a first forward traveling path of a vehicle on the basis of the obtained first information, wherein the first information is obtained from a radar sensor installed in the vehicle and includes at least distance measurement data indicating a distance to a first object and angle measurement data indicating an orientation of the first object, and wherein the first forward traveling path extends a specified distance in front of the vehicle and is a part of a forward traveling path to which the vehicle is going to travel; a second estimating means (40, S110, S120, S140) for obtaining second information and estimating a second radius of a second forward traveling path of the vehicle on the basis of the obtained second information, wherein the second information is obtained from the radar sensor installed in the vehicle and includes at least distance measurement data indicating a distance to a second object and angle measurement data indicating an orientation of the second object, and wherein the second forward traveling path is a part of the forward traveling path to which the vehicle is going to travel, the second forward traveling path being farther ahead of the vehicle than the first traveling path is, with the second forward traveling path having a forward end which is farther ahead of the own vehicle than a forward end of the first forward traveling path is; and a determining means (40, S150 to S180) for determining whether or not there is a changing point where road shapes change between the first forward traveling path and the second forward traveling path on the basis of comparison of the estimated first radius and the estimated second radius, characterized in that the first and second estimating means (40, S110 to S130, S140) is arranged to cluster the distance and angle measurement data, to recognize, for each cluster, a target existing ahead of the vehicle by calculating at least a central position coordinate of the target, a size of the target and a velocity of the target relative to the vehicle, to detect a type, such as a roadside object or a leading vehicle, of a respective target recognized from each cluster, and to estimate the first and second radius on the basis of a position of a target whose type has been detected; and the determining means (40, S150 to S180) is arranged to calculate an absolute value (ΔR) of the difference between the first radius and the second radius and configured to determine that there is no changing point when the absolute value (ΔR) is less than a predetermined threshold value (Th), and to determine that there is a changing point when the absolute value (ΔR) is equal to or larger than the predetermined threshold value (Th). a first estimating means (40, S110 to S130) for obtaining first information and estimating a first radius of a first forward traveling path of a vehicle on the basis of the obtained first information, wherein the first information is obtained from a radar sensor installed in the vehicle and includes at least distance measurement data indicating a distance to a first object and angle measurement data indicating an orientation of the first object, and wherein the first forward traveling path extends a specified distance in front of the vehicle and is a part of a forward traveling path to which the vehicle is going to travel; a second estimating means (40, S110, S120, S140) for obtaining second information and estimating a second radius of a second forward traveling path of the vehicle on the basis of the obtained second information, wherein the second information is obtained from the radar sensor installed in the vehicle and includes at least distance measurement data indicating a distance to a second object and angle measurement data indicating an orientation of the second object, and wherein the second forward traveling path is a part of the forward traveling path to which the vehicle is going to travel, the second forward traveling path being farther ahead of the vehicle than the first traveling path is, with the second forward traveling path having a forward end which is farther ahead of the own vehicle than a forward end of the first forward traveling path is; and a determining means (40, S150 to S180) for determining whether or not there is a changing point where road shapes change between the first forward traveling path and the second forward traveling path on the basis of comparison of the estimated first radius and the estimated second radius, characterized in that the first and second estimating means (40, S110 to S130, S140) is arranged to cluster the distance and angle measurement data, to recognize, for each cluster, a target existing ahead of the vehicle by calculating at least a central position coordinate of the target, a size of the target and a velocity of the target relative to the vehicle, to detect a type, such as a roadside object or a leading vehicle, of a respective target recognized from each cluster, and to estimate the first and second radius on the basis of a position of a target whose type has been detected; and the determining means (40, S150 to S180) is arranged to calculate an absolute value (ΔR) of the difference between the first radius and the second radius and configured to determine that there is no changing point when the absolute value (ΔR) is less than a predetermined threshold value (Th), and to determine that there is a changing point when the absolute value (ΔR) is equal to or larger than the predetermined threshold value (Th).