A method of communicating between a client and a service provider using an external server, the method comprising: an authentication procedure comprising: logging into the service provider; receiving from the service provider a first authentication token indicating that the service provider has authenticated communication with the client; and storing in the external server authentication information containing the first authentication token and additional information relating to communication with the service provider; and a communication procedure, when there is a request to access the service provider, the communication procedure comprising: receiving authentication information corresponding to the request from the external server; determining whether or not the first authentication token contained in the authentication information is valid, and in this case communicating with the service provider using the received authentication information from the external server without logging on to the service provider subsequent to the authentication procedure; and in the case where it is determined that the first authentication token is invalid, receiving a valid second authentication token from the service provider using the additional information contained in the authentication information, and communicating with the service provider using the valid second authentication token. an authentication procedure comprising: logging into the service provider; receiving from the service provider a first authentication token indicating that the service provider has authenticated communication with the client; and storing in the external server authentication information containing the first authentication token and additional information relating to communication with the service provider; and logging into the service provider; receiving from the service provider a first authentication token indicating that the service provider has authenticated communication with the client; and storing in the external server authentication information containing the first authentication token and additional information relating to communication with the service provider; and a communication procedure, when there is a request to access the service provider, the communication procedure comprising: receiving authentication information corresponding to the request from the external server; determining whether or not the first authentication token contained in the authentication information is valid, and in this case communicating with the service provider using the received authentication information from the external server without logging on to the service provider subsequent to the authentication procedure; and in the case where it is determined that the first authentication token is invalid, receiving a valid second authentication token from the service provider using the additional information contained in the authentication information, and communicating with the service provider using the valid second authentication token. receiving authentication information corresponding to the request from the external server; determining whether or not the first authentication token contained in the authentication information is valid, and in this case communicating with the service provider using the received authentication information from the external server without logging on to the service provider subsequent to the authentication procedure; and in the case where it is determined that the first authentication token is invalid, receiving a valid second authentication token from the service provider using the additional information contained in the authentication information, and communicating with the service provider using the valid second authentication token.