A data relay device for being provided between a display unit and a source device, the display unit comprising a first memory configured to store control data representing display attributes of the display unit, the data relay device comprising: a second memory configured to store the control data; a hot plug line connectable to the source device, which - when active - is indicative of a state of the data relay device in which the storing process of the control data in the second memory is terminated; a control unit configured to activate a power-on line connectable to the display device, to keep the hot plug line inactive from a time when the power-on line goes active and until the completion of storing the control data in the second memory, to activate the hot plug line and to keep the power-on line active after the control data is stored in the second memory, whereby the source device can read the control data from the second memory; a first data bus which is configured to transfer the control data from the display unit to the second memory when the data relay device is connected with the display unit; and a second data bus which is configured to transfer the read control data stored in the second memory to the source device when the data relay device is connected with the source device, and wherein the control unit has terminals (61 to 65) connected to the second data bus and which is configured to control the conduction/nonconduction of the second data bus for the transfer of the read control data stored in the second memory to the source device according to the presence or absence of access of the source device to the second memory when the data relay device is connected with the source device. a second memory configured to store the control data; a hot plug line connectable to the source device, which - when active - is indicative of a state of the data relay device in which the storing process of the control data in the second memory is terminated; a control unit configured to activate a power-on line connectable to the display device, to keep the hot plug line inactive from a time when the power-on line goes active and until the completion of storing the control data in the second memory, to activate the hot plug line and to keep the power-on line active after the control data is stored in the second memory, whereby the source device can read the control data from the second memory; a first data bus which is configured to transfer the control data from the display unit to the second memory when the data relay device is connected with the display unit; and a second data bus which is configured to transfer the read control data stored in the second memory to the source device when the data relay device is connected with the source device, and wherein the control unit has terminals (61 to 65) connected to the second data bus and which is configured to control the conduction/nonconduction of the second data bus for the transfer of the read control data stored in the second memory to the source device according to the presence or absence of access of the source device to the second memory when the data relay device is connected with the source device.