communication unit 73 attaches the chat room id to the supplied chat text and shared information and sends the chat text and the shared information via the network 1 to the server 2 the chat room manager 12 of the server 2 stores the chat text received from the client computer 3 2 in the chat log data management region for the chat room corresponding to the chat room id in the database 13 the chat room manager 12 reads the shared information id attached to the received shared information and determines whether or not shared information having the same shared information id is stored in the shared information management region as a result it is determined that the shared information id of the shared information previously stored in the shared information management region in step s 62 is the same as that of the currently received shared information the chat room manager 12 deletes the shared information having the same shared information id which has been stored in the shared information management region and instead stores the currently received shared information in the shared information management region subsequently the chat room manager 12 of the server 2 sends the chat log data stored in the chat log data management region and the shared information stored in the shared information management region to the client computers 3 1 and 3 2 the client computers 3 1 and 3 2 receive the chat log data and the shared information from the server 2 each chat log manager 68 stores the chat log data in the corresponding storage unit 72 and additionally displays the chat log data in.