Provided is a communication system capable of fundamentally preventing an attack from an unspecified counterpart and resolve problem even when a problem occurs in a user terminal or client and a server. A mediation server (1) receives an attack from an outside source on behalf of a server (34) (server A), redirects and receives communication addressed to a node-representative ID, and performs first authentication for the communication. In the case of a reliable user terminal or client (42) (client B), the mediation server (1) provides the device with an intermediary server ID specifying an intermediary server (2). The intermediary server (2) performs second authentication for the user terminal or client (42) (client B) going through the mediation server (1). When it is identified that the user terminal or client (42) (client B) is reliable, the intermediary server (2) asks the server (34) (server A) about an ID and provides the user terminal or client (42) (client B) with traceable ID information acquired from the server (34) (server A).