A running control system having a radar device that detects a distance between a subject vehicle and a preceding vehicle. When a vehicle speed sensor detects the subject vehicle has stopped, a transmission output controller sets a transmission output of the radar device to a stopping time transmission output lower than a running time transmission output. When the radar device detects an increase in the distance between the subject vehicle and the preceding vehicle while the subject vehicle is stopped, or when the distance between the subject vehicle and the preceding vehicle detected by the radar device while the subject vehicle has stopped is longer than a predetermined value, a notifying device notifies an occupant of the subject vehicle that following running control is possible.