A time synchronization system (1, 1A, 1B, 1C, 2) comprising: a reference time obtaining unit configured to obtain, based on a radio signal from a global navigation satellite system, a first timing signal and time information corresponding to timing indicated by the first timing signal; and a modulator, connected to a line branched into a plurality of branches, configured to generate a modulated signal containing time information in synchronization with the first timing signal and to send the modulated signal over the line, and the time synchronization system further comprising: at least one demodulator, connected to any branch of the line, configured to demodulate the modulated signal propagating over the line, and at least one transmitter configured to transmit a first radio signal compatible with the radio signal from the global navigation satellite system based on a second timing signal and the time information obtained through demodulation by any demodulator, wherein the modulator is configured to receive the first timing signal periodically, and send the modulated signal over the line at a timing determined from a time point of reception of the first timing signal, as a reference, and the modulated signal contains a synchronization word in addition to the time information, and wherein the demodulator is configured to output, when the demodulator detects the synchronization word contained in the modulated signal propagating over the line, information following the detected synchronization word as demodulated data and to output the second timing signal at a timing determined from a time point preceding a time point of detection of the synchronization word by predetermined correction time, as a reference. a reference time obtaining unit configured to obtain, based on a radio signal from a global navigation satellite system, a first timing signal and time information corresponding to timing indicated by the first timing signal; and a modulator, connected to a line branched into a plurality of branches, configured to generate a modulated signal containing time information in synchronization with the first timing signal and to send the modulated signal over the line, and the time synchronization system further comprising: at least one demodulator, connected to any branch of the line, configured to demodulate the modulated signal propagating over the line, and at least one transmitter configured to transmit a first radio signal compatible with the radio signal from the global navigation satellite system based on a second timing signal and the time information obtained through demodulation by any demodulator, wherein the modulator is configured to receive the first timing signal periodically, and send the modulated signal over the line at a timing determined from a time point of reception of the first timing signal, as a reference, and receive the first timing signal periodically, and send the modulated signal over the line at a timing determined from a time point of reception of the first timing signal, as a reference, and the modulated signal contains a synchronization word in addition to the time information, and wherein the demodulator is configured to output, when the demodulator detects the synchronization word contained in the modulated signal propagating over the line, information following the detected synchronization word as demodulated data and to output the second timing signal at a timing determined from a time point preceding a time point of detection of the synchronization word by predetermined correction time, as a reference.