A communication system comprising: an antenna that is configured to transmit and receive an electric signal to and from an object for communication in an exterior of a structure; a device in an interior of the structure, wherein the device includes an interior antenna that is provided in the interior and is configured to transmit and receive an electric signal; characterized by the antenna being configured to transmit and receive the electric signal at a high frequency of 6 Ghz to 90 Ghz; a converter connected to the antenna and configured to convert the electric signal received by the antenna into an optical signal; an optical fiber configured to connect the converter and the device, wherein the optical fiber is a multimode optical fiber having a plurality of propagation modes for propagation of optical signals; an optical coupler connected to the multimode optical fiber configured to branch the optical signal propagated through the multimode optical fiber; and a transferring unit configured to be connected to a control box and to transfer an electric signal to and from the control box provided in the interior, wherein the device further includes a first opto-electric converter (21d, 21m) and a second opto-electric converter (21d, 21n), wherein the first opto-electric converter (21d, 21m) is connected to the optical coupler and the transferring unit, and is configured to convert the optical signal propagated through the multimode optical fiber and branched by the optical coupler into the electric signal, and to output the converted electric signal to the transferring unit, and the second opto-electric converter (21d, 21n) is connected to the optical coupler and the interior antenna, and is configured to convert the optical signal propagated through the multimode optical fiber and branched by the optical coupler into the electric signal, and to output the converted electric signal to the interior antenna. an antenna that is configured to transmit and receive an electric signal to and from an object for communication in an exterior of a structure; a device in an interior of the structure, wherein the device includes an interior antenna that is provided in the interior and is configured to transmit and receive an electric signal; characterized by the antenna being configured to transmit and receive the electric signal at a high frequency of 6 Ghz to 90 Ghz; a converter connected to the antenna and configured to convert the electric signal received by the antenna into an optical signal; an optical fiber configured to connect the converter and the device, wherein the optical fiber is a multimode optical fiber having a plurality of propagation modes for propagation of optical signals; an optical coupler connected to the multimode optical fiber configured to branch the optical signal propagated through the multimode optical fiber; and a transferring unit configured to be connected to a control box and to transfer an electric signal to and from the control box provided in the interior, wherein the device further includes a first opto-electric converter (21d, 21m) and a second opto-electric converter (21d, 21n), wherein the first opto-electric converter (21d, 21m) is connected to the optical coupler and the transferring unit, and is configured to convert the optical signal propagated through the multimode optical fiber and branched by the optical coupler into the electric signal, and to output the converted electric signal to the transferring unit, and the second opto-electric converter (21d, 21n) is connected to the optical coupler and the interior antenna, and is configured to convert the optical signal propagated through the multimode optical fiber and branched by the optical coupler into the electric signal, and to output the converted electric signal to the interior antenna. the first opto-electric converter (21d, 21m) is connected to the optical coupler and the transferring unit, and is configured to convert the optical signal propagated through the multimode optical fiber and branched by the optical coupler into the electric signal, and to output the converted electric signal to the transferring unit, and the second opto-electric converter (21d, 21n) is connected to the optical coupler and the interior antenna, and is configured to convert the optical signal propagated through the multimode optical fiber and branched by the optical coupler into the electric signal, and to output the converted electric signal to the interior antenna.