In an automatic train stop system in which a track circuit (3) is divided into track circuit sections (1T; 2T; and 3T), and a digitized train control information telegram (ATS telegram) (8) is transmitted through the track circuit sections on carriers having different frequencies, a carrier sensor (12) mounted on a train (5) receives a carrier in the track circuit (3) and detects a track circuit boundary (31) by detecting a level change of the carrier, whereby an onboard system grasps the position thereof and controls the train (5) based on the information of the ATS telegram (8).