We investigated the progress of the safety respects of ATS (Automatic Train Stop) system which is introduced in almost all narrow-gauge lines of JR. The ATS system consists of an on-board antenna and beacons, and intermittently transmits only the resonance frequency signal from beacons to on-board equipment. So we are developing a new ATS system with a function to overlay digital codes having information about the distance to stop signal on the resonance frequency signal transmitted from beacons in use. In this paper, we report the selection of the frequency for transmitting digital codes, the feature of proposed system and the results of laboratory tests.