An apparatus for transmitting terrestrial digital data using a synchronous-optical-network system or a synchronous-digital-hierarchy system includes a synchronous-difference calculating unit that calculates a synchronous difference between a standard signal used in the synchronous-optical-network system or the synchronous-digital-hierarchy system and a first synchronous signal that synchronizes with a leading position included in image data of the terrestrial digital data; and a transmission processing unit that transmits the synchronous difference together with the terrestrial digital data to a destination.