A message transmission system includes a registration accepting part for accepting a membership number, a subscriber telephone number, and a subscribed service ID; a subscriber information storage for storing membership numbers, subscriber telephone numbers, and subscribed service IDs in association with each other; a transmission request receiver for receiving a membership number of a message destination and a service ID of a Web server from the Web server or from communication terminal; a communication address converter for referring to the subscriber information storage on the basis of the received membership number and service ID to retrieve the subscriber telephone number corresponding to the membership number and service ID; and a message output part for outputting a predetermined message while designating the retrieved subscriber telephone number.