A display data sharing system, comprising a first communication terminal, second communication terminal and an external input apparatus connected with the first communication terminal, the first communication terminal delivering conference data including voice or video to the second communication terminal through a network, the external input apparatus comprising: a first display part configured to display an image; a display data obtaining part configured to obtain display data of the image displayed on the first display part; a display data transmission part for transmitting the display data obtained by the display data obtaining part to the first communication terminal, the external input apparatus being configured to control the display data transmission part to transmit the display data to the second communication terminal by using the first communication terminal; a reception part configured to receive, from the first communication terminal, display control information indicating a display data shared state if the second communication terminal is receiving the display data transmitted by the display data transmission part and therefore shares the display data with the first communication terminal or a display data unshared state if the display data is not shared between the first communication terminal and the second communication terminal; and a menu control part configured to update a menu to be displayed on the first display part based on the display control information received by the reception part, the menu being for indicating whether the display data shared state or the display data unshared state is present; wherein the first communication terminal comprises an operation input reception part for receiving a request to start remote communication with the second communication terminal and wherein the external input apparatus further comprises a connection detection part for detecting that data transmission with the second communication terminal by using the first communication terminal is possible and for causing the first display part to display a menu for receiving a user's operation to start sharing the display data with the second communication terminal and an operation input reception part for receiving, from the menu, the user's operation to start sharing the display data with the second communication terminal, wherein the display data obtaining part is configured to obtain the display data when the operation input reception part for receiving the user's operation receives the user's operation to start sharing the display data with the second communication terminal, wherein the display data transmission part transmits the display data obtained by the display data obtaining part to the second communication terminal by using the first communication terminal when the display data obtaining part obtains the display data and when the connection detection part has detected that data transmission with the second communication terminal is possible. a first display part configured to display an image; a display data obtaining part configured to obtain display data of the image displayed on the first display part; a display data transmission part for transmitting the display data obtained by the display data obtaining part to the first communication terminal, the external input apparatus being configured to control the display data transmission part to transmit the display data to the second communication terminal by using the first communication terminal; a reception part configured to receive, from the first communication terminal, display control information indicating a display data shared state if the second communication terminal is receiving the display data transmitted by the display data transmission part and therefore shares the display data with the first communication terminal or a display data unshared state if the display data is not shared between the first communication terminal and the second communication terminal; and a menu control part configured to update a menu to be displayed on the first display part based on the display control information received by the reception part, the menu being for indicating whether the display data shared state or the display data unshared state is present; wherein the first communication terminal comprises an operation input reception part for receiving a request to start remote communication with the second communication terminal and wherein the external input apparatus further comprises a connection detection part for detecting that data transmission with the second communication terminal by using the first communication terminal is possible and for causing the first display part to display a menu for receiving a user's operation to start sharing the display data with the second communication terminal and an operation input reception part for receiving, from the menu, the user's operation to start sharing the display data with the second communication terminal, wherein the display data obtaining part is configured to obtain the display data when the operation input reception part for receiving the user's operation receives the user's operation to start sharing the display data with the second communication terminal, wherein the display data transmission part transmits the display data obtained by the display data obtaining part to the second communication terminal by using the first communication terminal when the display data obtaining part obtains the display data and when the connection detection part has detected that data transmission with the second communication terminal is possible.