An optical network system comprising: a master unit; a plurality of gateway units (5a, 5b, 5c) connected to the master unit; and an electronic control unit connected to each of the plurality of gateway units (5a, 5b, 5c), wherein the master unit includes a light source, a signal processing unit, a network control unit, and a photodetector, the signal processing unit is configured to control the light source to generate an optical signal containing an information portion to be read by any one of the plurality of gateway units (5a, 5b, 5c) and a continuous light portion to be written by any one of the plurality of gateway units (5a, 5b, 5c), the network control unit is configured to designate any one of the plurality of gateway units (5a, 5b, 5c) and to generate an electrical signal regarding whether to read or write information loaded on the optical signal from the light source, the plurality of gateway units (5a, 5b, 5c) are configured to receive the optical signal and the electrical signal, each of the gateway units (5a, 5b, 5c), when designated by the electrical signal, is configured to read information contained in a corresponding optical signal which corresponds to the electrical signal, or to write information to the continuous light portion in the corresponding optical signal, and to exchange information with the electronic control unit on the basis of information regarding whether to read or write information contained in the electrical signal, and the photodetector is configured to read the information written to the continuous light portion by any one of the gateway units (5a, 5b, 5c). a master unit; a plurality of gateway units (5a, 5b, 5c) connected to the master unit; and an electronic control unit connected to each of the plurality of gateway units (5a, 5b, 5c), wherein the master unit includes a light source, a signal processing unit, a network control unit, and a photodetector, the signal processing unit is configured to control the light source to generate an optical signal containing an information portion to be read by any one of the plurality of gateway units (5a, 5b, 5c) and a continuous light portion to be written by any one of the plurality of gateway units (5a, 5b, 5c), the network control unit is configured to designate any one of the plurality of gateway units (5a, 5b, 5c) and to generate an electrical signal regarding whether to read or write information loaded on the optical signal from the light source, the plurality of gateway units (5a, 5b, 5c) are configured to receive the optical signal and the electrical signal, each of the gateway units (5a, 5b, 5c), when designated by the electrical signal, is configured to read information contained in a corresponding optical signal which corresponds to the electrical signal, or to write information to the continuous light portion in the corresponding optical signal, and to exchange information with the electronic control unit on the basis of information regarding whether to read or write information contained in the electrical signal, and the photodetector is configured to read the information written to the continuous light portion by any one of the gateway units (5a, 5b, 5c).