The AAA client generates accounting data, transmits an accounting data transmission request message to the AAA server, and then receives a response message to the accounting data transmission request message from the AAA server. If receiving a transmission failure response message for the accounting data from the AAA server, the AAA client stores accounting data generated after receiving the transmission failure response message, and if an amount of the stored accounting data is increased to a certain limit, the AAA client sets an identifier for a batch accounting application in the accounting data and transmits an accounting data transmission request message, including stored batch accounting data and the session information, to the AAA server. The AAA server confirms the identifier and the transmission request message and searches the accounting record for session information mapped to the session information included in the ACR message.