Data indicating a transfer mode of a transmission packet, as well as data indicating a timing mode of a transmission packet, is inserted in a payload unit contained in a transmission packet of a serial digital transfer interface (SDTI). A transfer mode includes, in addition to an asynchronous transfer mode, a synchronous transfer mode and an isochronous transfer mode. A timing mode which specifies with which field of a transmission frame a transmission packet is to be synchronized in transferring the transmission packet, includes a normal mode using a first field, an advanced mode using a second field and a dual mode using both fields. A system delay between transmission and reception can be minimized by a proper setting of a timing mode.