A transmission management system that manages transmission of image data and sound data between a first transmission terminal and a second transmission terminal via a relay device, the transmission management system comprising: a data transmitter/receiver that receives delay information indicating delay in time at which the image data is received at the second transmission terminal through the relay device, from the second transmission terminal, characterized by the system further comprising: a message generator that determines whether the relay device interrupts relaying of the image data received from the first transmission terminal to the second transmission terminal based on image state information that is associated with the received delay information to generate a determination result, the image state information indicating whether the relay device interrupts transmission of the image data to the second transmission terminal when a delay time specified by the received delay information is obtained, wherein when the determination result indicates that the relay device interrupts relaying of the image data, the message generator generates a message indicating that the image data is not received or only sound data is received, and transmits the message to the second transmission terminal through the data transmitter/receiver for display at the second transmission terminal. a data transmitter/receiver that receives delay information indicating delay in time at which the image data is received at the second transmission terminal through the relay device, from the second transmission terminal, characterized by the system further comprising: a message generator that determines whether the relay device interrupts relaying of the image data received from the first transmission terminal to the second transmission terminal based on image state information that is associated with the received delay information to generate a determination result, the image state information indicating whether the relay device interrupts transmission of the image data to the second transmission terminal when a delay time specified by the received delay information is obtained, wherein when the determination result indicates that the relay device interrupts relaying of the image data, the message generator generates a message indicating that the image data is not received or only sound data is received, and transmits the message to the second transmission terminal through the data transmitter/receiver for display at the second transmission terminal. a message generator that determines whether the relay device interrupts relaying of the image data received from the first transmission terminal to the second transmission terminal based on image state information that is associated with the received delay information to generate a determination result, the image state information indicating whether the relay device interrupts transmission of the image data to the second transmission terminal when a delay time specified by the received delay information is obtained, wherein when the determination result indicates that the relay device interrupts relaying of the image data, the message generator generates a message indicating that the image data is not received or only sound data is received, and transmits the message to the second transmission terminal through the data transmitter/receiver for display at the second transmission terminal.