An optical transmission system (S, Sb, Sc, Sd, Se, Sf) including an optical transmitting apparatus (1b, 1c, 1r, 1t, 1ta) and an optical receiving apparatus (2b, 2c, 2f, 2r, 2ra, 2t) that receives, via an optical transmission line (3, 3c), a signal transmitted from the optical transmitting apparatus (1b, 1c, 1r, 1t, 1ta), the optical transmission system (S, Sb, Sc, Sd, Se, Sf) comprising: one or more transmission-mode selectors (60, 60c, 103t, 203r, 203t) configured to select transmission mode information in descending order of priority out of transmission mode information, which is a plurality of kinds of the transmission mode information common to transmission performance of the optical transmitting apparatus (1b, 1c, 1r, 1t, 1ta) and the optical receiving apparatus (2b, 2c, 2f, 2r, 2ra, 2t) obtained by each of the optical transmitter and receiver informing each other of the available transmission mode information, the transmission mode information being combinations of a plurality of parameters concerning the transmission performance including at least a modulation scheme, a baud rate, and an error correction code type; a signal transmitter(110t, 110ta) configured to transmit, to the optical receiving apparatus (2b, 2c, 2f, 2r, 2ra, 2t), a signal modulated based on the selected transmission mode information; and a signal receiver configured to receive the signal and demodulates the received signal based on the transmission mode information selected by the one or more transmission-mode selectors (60, 60c, 103t, 203r, 203t), wherein the one or more transmission-mode selectors (60, 60c, 103t, 203r, 203t) is configured to output selected transmission mode information to the optical transmitting apparatus (1b, 1c, 1r, 1t, 1ta) and the optical receiving apparatus (2b, 2c, 2f, 2r, 2ra, 2t). one or more transmission-mode selectors (60, 60c, 103t, 203r, 203t) configured to select transmission mode information in descending order of priority out of transmission mode information, which is a plurality of kinds of the transmission mode information common to transmission performance of the optical transmitting apparatus (1b, 1c, 1r, 1t, 1ta) and the optical receiving apparatus (2b, 2c, 2f, 2r, 2ra, 2t) obtained by each of the optical transmitter and receiver informing each other of the available transmission mode information, the transmission mode information being combinations of a plurality of parameters concerning the transmission performance including at least a modulation scheme, a baud rate, and an error correction code type; a signal transmitter(110t, 110ta) configured to transmit, to the optical receiving apparatus (2b, 2c, 2f, 2r, 2ra, 2t), a signal modulated based on the selected transmission mode information; and a signal receiver configured to receive the signal and demodulates the received signal based on the transmission mode information selected by the one or more transmission-mode selectors (60, 60c, 103t, 203r, 203t), wherein the one or more transmission-mode selectors (60, 60c, 103t, 203r, 203t) is configured to output selected transmission mode information to the optical transmitting apparatus (1b, 1c, 1r, 1t, 1ta) and the optical receiving apparatus (2b, 2c, 2f, 2r, 2ra, 2t).