To overcome such a problem that a known character-based communication means has the defect that the security of a mail sender is not ensured and requires special software and the load on the CPUs, memories, etc., of a server and an accessing personal computer is heavy.

The purpose is attained by a database system based principally on DCBid's, message id's, and user id's as keys, and a communication means which adopts a style wherein a sender or inviter dynamically generates a place (DCB, DRB) for communication and the producer invites participants or applicants, ensures the security of message senders, and allows a closed communication with only selected partners; and the security level is improved by using a means for ciphering, etc., by SSL, etc., and uses of even business become possible.