The purpose of this paper was to develop a triangular surveying method based on the time of flight between the acoustic beacon and the single microphone with the aim of improving indoor localization services. In the proposed method, plural amplitude modulated signals were applied as the beacon signal to reduce the effect of disturbing noises, and direct conversion receivers were used to cancel the difference of synchronization between the transmitter and the receiver. The performance of the proposed method was evaluated by moving the microphone between a pair of acoustic beacons which were not synchronized to the receiver. From the results of the evaluation, the utility of the proposed method was confirmed in most of the ranges. This method could be a major step forward in improving indoor localization services.