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A history information storage unit accumulates an interaction history which corresponds to each interaction session performed by a communication terminal and which at least contains the other-party user ID and the other-party terminal ID. If User A, who is interacting with the terminal b1 of User B, sends from the terminal 1 a an interaction history request containing the ID of User B and the IDs of the other terminals a2 to an owned by User A, a history information computation unit searches, from the history information storage unit, the interaction histories for each interaction session respectively corresponding to the IDs of terminals a2 to an specified in the interaction history request to retrieve and send to the requesting terminal 1 a interaction history containing the other-party user identifier matching the ID of User B.