A transmission control system configured to control a plurality of transmission terminals (10aa; 10bb), each of the transmission terminals (10aa; 10bb) transmitting and receiving content data via a certain relay device (30a; 30b), the transmission control system comprising: a storage unit configured to store therein zone identification information for identifying a zone (A; B) in which each of the transmission terminals (10aa; 10bb) is used, and priority information that indicates a priority at which a transmission terminal (10aa; 10bb) indicated by the zone identification information should use each of a plurality of relay devices (30a; 30b), in association with each other; a reception unit configured to receive, from a first transmission terminal that requests initiation of communication with a second transmission terminal as a destination terminal, initiation request information that includes a terminal ID of the first transmission terminal, the first terminal ID including first zone identification information for identifying a zone in which the first transmission terminal is used, and a second terminal ID of the second transmission terminal, the second terminal ID including second zone identification information for identifying a zone in which the second transmission terminal as the destination terminal is used; and a selection unit configured to select a relay device (30a; 30b) to be used for relaying the content data based on first priority information associated with the first zone identification information in the storage unit and second priority information associated with the second zone identification information in the storage unit, the priority indicating a value calculated based on round trip time between a relay device corresponding to a zone in which each of the transmission terminals (10aa; 10bb) is used, and a relay device corresponding to another zone different from the zone. a storage unit configured to store therein zone identification information for identifying a zone (A; B) in which each of the transmission terminals (10aa; 10bb) is used, and priority information that indicates a priority at which a transmission terminal (10aa; 10bb) indicated by the zone identification information should use each of a plurality of relay devices (30a; 30b), in association with each other; a reception unit configured to receive, from a first transmission terminal that requests initiation of communication with a second transmission terminal as a destination terminal, initiation request information that includes a terminal ID of the first transmission terminal, the first terminal ID including first zone identification information for identifying a zone in which the first transmission terminal is used, and a second terminal ID of the second transmission terminal, the second terminal ID including second zone identification information for identifying a zone in which the second transmission terminal as the destination terminal is used; and a selection unit configured to select a relay device (30a; 30b) to be used for relaying the content data based on first priority information associated with the first zone identification information in the storage unit and second priority information associated with the second zone identification information in the storage unit, the priority indicating a value calculated based on round trip time between a relay device corresponding to a zone in which each of the transmission terminals (10aa; 10bb) is used, and a relay device corresponding to another zone different from the zone.