A PON, Passive Optical Network, system comprising an OLT, Optical Line Terminal,, an optical branching device and a plurality of ONUs, Optical Network Units, (500 to 502) connected to the OLT through the optical branching device, where data is transmitted from the plurality of ONUs (500 to 502) to the OLT by time-division multiplexing, wherein at least one first ONU among the plurality of ONUs (500 to 502) includes: an emergency call detection means for detecting an emergency call request from a terminal device connected to the first ONU; and an emergency call origination control means for outputting, to the optical branching device, a disconnection request for disconnecting all ONUs (501, 502) other than the first ONU from the OLT when the emergency call request is detected by the emergency call detection means and connection of the first ONU to the OLT is not established, and the optical branching device includes: an optical splitter configured to split a downstream signal from the OLT into a plurality of optical transmission paths (400 to 402) respectively corresponding to the plurality of ONUs (500 to 502), and perform time-division multiplexing of upstream signals from the plurality of ONUs (500 to 502) to the OLT; a plurality of switches (310 to 312) respectively placed on the plurality of optical transmission paths (400 to 402); and an optical line control means for disconnecting all ONUs (501, 502) other than the first ONU from the OLT in response to the disconnection request from the emergency call origination control means, and wherein the optical line control means controls the plurality of switches (310 to 312) to disconnect all ONUs (501, 502) other than the first ONU from the OLT and connect the first ONU to the OLT in response to the disconnection request. an emergency call detection means for detecting an emergency call request from a terminal device connected to the first ONU; and an emergency call origination control means for outputting, to the optical branching device, a disconnection request for disconnecting all ONUs (501, 502) other than the first ONU from the OLT when the emergency call request is detected by the emergency call detection means and connection of the first ONU to the OLT is not established, and the optical branching device includes: an optical splitter configured to split a downstream signal from the OLT into a plurality of optical transmission paths (400 to 402) respectively corresponding to the plurality of ONUs (500 to 502), and perform time-division multiplexing of upstream signals from the plurality of ONUs (500 to 502) to the OLT; a plurality of switches (310 to 312) respectively placed on the plurality of optical transmission paths (400 to 402); and an optical line control means for disconnecting all ONUs (501, 502) other than the first ONU from the OLT in response to the disconnection request from the emergency call origination control means, and wherein the optical line control means controls the plurality of switches (310 to 312) to disconnect all ONUs (501, 502) other than the first ONU from the OLT and connect the first ONU to the OLT in response to the disconnection request. an optical splitter configured to split a downstream signal from the OLT into a plurality of optical transmission paths (400 to 402) respectively corresponding to the plurality of ONUs (500 to 502), and perform time-division multiplexing of upstream signals from the plurality of ONUs (500 to 502) to the OLT; a plurality of switches (310 to 312) respectively placed on the plurality of optical transmission paths (400 to 402); and an optical line control means for disconnecting all ONUs (501, 502) other than the first ONU from the OLT in response to the disconnection request from the emergency call origination control means, and wherein the optical line control means controls the plurality of switches (310 to 312) to disconnect all ONUs (501, 502) other than the first ONU from the OLT and connect the first ONU to the OLT in response to the disconnection request.