A biological information monitoring system includes: a measuring circuitry configured acquire information on a potential difference between two first electrodes that is placed on a first thorax of a user, wherein the first thorax is in a symmetric position to a second thorax that is positioned at the same side as a heart of the user; a detection circuitry configured to detect a plurality of S wave peaks based on the information on the potential difference to generate time-series information on the plurality of S wave peaks; and a processing circuitry configured to determine respiratory information on the user based on the time-series information on the plurality of S wave peaks, and to output the respiratory information as biological information.