Provided is a digital-image transmission apparatus including a transmitting unit connected to a host device and a receiving unit connected to a display device. The transmitting unit includes a host-side additional-communication control unit that controls additional communication for digital image transmission. The host-side additional-5 communication control unit performs communication with an external device to diagnose an operation state of the transmitting unit, transmit a diagnosis result to the external device, and control an operation of the transmitting unit.