A transmission system is accessible to association information that associates, for each one of one or more sessions, terminal identification information for identifying each one of a plurality of transmission terminals that are currently participating in the session, with reservation identification information for identifying reservation of communication on which the session is based. The transmission system obtains reservation identification information associated with the counterpart transmission terminal using the association information and counterpart terminal identification information. The transmission system determines whether reservation identification information associated with the request transmission terminal matches the reservation identification information associated with the counterpart transmission terminal to generate a determination result. Based on the determination result, a request for starting communication with the counterpart transmission terminal is accepted or rejected.