Provided is a digital image transmission system in a high definition multimedia interface (HDMI) format or a digital visual interface (DVI) format. A transmission and reception reinforcement device of the digital image transmission system includes a transmitter, a receiver, and a power supply circuit. The power supply circuit includes a first power supply unit supplying power to the transmitter and a second power supply unit supplying power to the receiver. Power output terminals of the first and second power supply units are connected to each other. External-power supply voltages respectively input to the first and second power supply units are output to the power output terminals. When all of the external-power supply voltages input to the first and second power supply units are lower than a predetermined voltage, an internal-power supply voltage present in a communication line between the host device and the display device is output to the power output terminals.