Disclosed is a transmission system including a first transmission terminal and a second transmission terminal in which the first transmission terminal transmits data to the second transmission terminal. The transmission system includes a first communications connector configured to connect first data communications between the first transmission terminal specified by first identification information and the second transmission terminal specified by second identification information, a determination part configured to refer to registered information registering third identification information in association with the first identification information of the first transmission terminal, acquire the third identification information in association with the first identification information, and determine whether the third identification information is included in transmission data transmitted by the first transmission terminal, and an addition part configured to add first information indicating that determination is made on the transmission data by the determination part.