A method of controlling display of image data, comprising: receiving (S83, S94) image data respectively from one or more counterpart transmission terminals via a relay device; displaying one or more images based on the image data that are respectively received from the one or more counterpart transmission terminals on a display; receiving (S123-1) message data from a management system in alternative to image data to be transmitted from one of the one or more counterpart transmission terminals; displaying (S123-3) a message on the display based on the received message data; displaying at least a part of the one or more images, respectively, on one or more display sections of the display based on the image data that are respectively received from the at least a part of the one or more counterpart transmission terminals; and: managing image data management information, which associates, for each one of at least the part of the one or more images, display section identification information for identifying the display section, with terminal identification information for identifying the counterpart transmission terminal that transmits the image data for display on the display section and which further associates, for each one of the remaining part of the one or more images, virtual section identification information for identifying a virtual section that is not displayed on the display, with terminal identification information for identifying the counterpart transmission terminal that transmits the image data; characterized in that the displaying step displays the message based on the received message data on a section on the display other than the display sections if the received terminal identification information for identifying the one of the one or more counterpart transmission terminals for which the message data is received in alternative to the image data is associated with the virtual section identification information. receiving (S83, S94) image data respectively from one or more counterpart transmission terminals via a relay device; displaying one or more images based on the image data that are respectively received from the one or more counterpart transmission terminals on a display; receiving (S123-1) message data from a management system in alternative to image data to be transmitted from one of the one or more counterpart transmission terminals; displaying (S123-3) a message on the display based on the received message data; displaying at least a part of the one or more images, respectively, on one or more display sections of the display based on the image data that are respectively received from the at least a part of the one or more counterpart transmission terminals; and: managing image data management information, which associates, for each one of at least the part of the one or more images, display section identification information for identifying the display section, with terminal identification information for identifying the counterpart transmission terminal that transmits the image data for display on the display section and which further associates, for each one of the remaining part of the one or more images, virtual section identification information for identifying a virtual section that is not displayed on the display, with terminal identification information for identifying the counterpart transmission terminal that transmits the image data; characterized in that the displaying step displays the message based on the received message data on a section on the display other than the display sections if the received terminal identification information for identifying the one of the one or more counterpart transmission terminals for which the message data is received in alternative to the image data is associated with the virtual section identification information. managing image data management information, which associates, for each one of at least the part of the one or more images, display section identification information for identifying the display section, with terminal identification information for identifying the counterpart transmission terminal that transmits the image data for display on the display section and which further associates, for each one of the remaining part of the one or more images, virtual section identification information for identifying a virtual section that is not displayed on the display, with terminal identification information for identifying the counterpart transmission terminal that transmits the image data; characterized in that the displaying step displays the message based on the received message data on a section on the display other than the display sections if the received terminal identification information for identifying the one of the one or more counterpart transmission terminals for which the message data is received in alternative to the image data is associated with the virtual section identification information.