Abstract: A digital data transmission apparatus which can suitably route and transmit to a desired apparatus two types digital data, that is, digital data of a serial digital interface (SDI) format and a serial digital data interface (SDDI) format having a partially common format to the SDI format even if they are mixed, the digital data transmission apparatus having a first routing apparatus (20, 20A) having a first signal supplying means (102) for supplying the data of a first transmission packet of the SDI format; a second signal supplying means (104) for supplying data of a second transmission packet of the SDDI format having a common control data portion to that of the SDI format and having the same data length; a first input terminal receiving the data of the first transmission packet; and a second input terminal receiving the data of the second transmission packet and outputs the first input data supplied to the first input terminal from the first output terminal and outputs the second data supplied to the secon