An ACC system includes a radar device for detecting an object in front of a subject vehicle, locus estimator for estimating a travel locus of the subject vehicle, detection area setter for setting a predetermined detection area in front of the subject vehicle based on the travel locus of the subject vehicle, obstacle extractor for extracting a preceding vehicle based on a detection result of the radar device and the detection area, and acceleration/deceleration intention detector for detecting an intention of an occupant to accelerate or decelerate. Since the detection area setter increases the detection area for an obstacle when the acceleration/deceleration intention detector detects an intention of the occupant to decelerate, it is possible to reliably detect a cutting-in vehicle that cuts in front of the subject vehicle and to prevent the subject vehicle from coming into contact with the cutting-in vehicle, thus preventing the driver from feeling uncomfortable and ensuring reliable operation of the ACC system.