12. A system according to claim 10, wherein when the message is posted, said server adds an identification code to a quotation sentence which cites said posted message, posts the message into said database, and specifies a duplicated portion of the message from said series of messages by using said identification code.
15. A system according to claim 12, wherein when the message transmitted from said client is posted into said database, said server posts the message into a message list and stores information for related subject with the other messages into said message list, and wherein said client has display means, said display means displays a message tree based on said message list, and when responding to the message displayed in said message tree by an instruction from the user, said display means adds said identification code to said quotation sentence from the message posted in said database and displays.