1. A content distributing system for transmitting content from a transmission device to a reception device via a routing device, the system comprising: the transmission device operable to generate additional information of content according to a property of the content, the additional information indicating whether or not to log communication of the content when the content is transmitted from the routing device to the reception device, and transmit the additional information, a transmission target address specifying the reception device, and the content, to the routing device; the routing device operable to receive the additional information, the transmission target address, and the content from the transmission device, and transmit the content to the reception device specified by the transmission target address, and when the additional information indicates to log the communication, generate log information indicating that the content has been routed from the transmission device to the reception device, and store the log information; the reception device operable to receive the content transmitted from the routing device; and an analyzing device operable to obtain the log information from the routing device, and judge whether the reception device is an unauthorized device, based upon the obtained log information.