A time synchronization method for a communication system configured to perform protection switching, the communication system including a first network in which a master station apparatus, comprises Optical Line Terminals, OLTs (,), and the first network is further including an Optical Network Unit, ONU which are connected via a plurality of physical lines including a working line and a backup line, and the communication system further comprises a second network connected to the ONU, the time synchronization method comprising: a step in which the master station apparatus transmits a synchronization signal to the ONU and the ONU synchronizes a clock of the ONU with a clock of the master station apparatus using the received synchronization signal and records information concerning a transmission source of the synchronization signal; a step in which the master station apparatus transmits a synchronization command including designated time information, a time stamp and a transmission source identifier Id that can identify an OLT and to the ONU where the designated time information is a time information received from a grand master and which is corrected using a processing time of the master station apparatus and delay times; a switching step for performing a protection switching in which the master station apparatus uses, when detecting a communication failure of an uplink signal in the working line, the backup line as a new working line instead of the working line; and a time synchronization step in which the ONU compares the information concerning a transmission source of the synchronization signal and the transmission source identifier Id designated in the synchronization command and executes, when the transmission source identifier Id and the information concerning the transmission source of the synchronization signal coincide with each other (ST30; S24a), synchronization processing for correcting the time information included in the synchronization command based on the clock of the ONU and the time stamp and transmitting the synchronization command to the second network; and when the transmission source identifier Id and the information concerning the transmission source of the synchronization signal do not coincide with each other, the ONU suppresses the synchronization processing based on the synchronization command and does not transmit time information. a step in which the master station apparatus transmits a synchronization signal to the ONU and the ONU synchronizes a clock of the ONU with a clock of the master station apparatus using the received synchronization signal and records information concerning a transmission source of the synchronization signal; a step in which the master station apparatus transmits a synchronization command including designated time information, a time stamp and a transmission source identifier Id that can identify an OLT and to the ONU where the designated time information is a time information received from a grand master and which is corrected using a processing time of the master station apparatus and delay times; a switching step for performing a protection switching in which the master station apparatus uses, when detecting a communication failure of an uplink signal in the working line, the backup line as a new working line instead of the working line; and a time synchronization step in which the ONU compares the information concerning a transmission source of the synchronization signal and the transmission source identifier Id designated in the synchronization command and executes, when the transmission source identifier Id and the information concerning the transmission source of the synchronization signal coincide with each other (ST30; S24a), synchronization processing for correcting the time information included in the synchronization command based on the clock of the ONU and the time stamp and transmitting the synchronization command to the second network; and when the transmission source identifier Id and the information concerning the transmission source of the synchronization signal do not coincide with each other, the ONU suppresses the synchronization processing based on the synchronization command and does not transmit time information.