To attain conversion of a signal series suitable for digital transmission attended with error control by even on the occurrence of an error in excess of an error correction capability of an error correction code in a digital transmission signal.

This digital transmission system adopts 27 (=128) QAM (quadrature amplitude modulation) demodulation system and employs a reed Solomon code on a GF (28) for its error correction system. The transmitter side applies 8-7 conversion to a transmission digital signal after error correction coding and applies modulation the 8-7 converted signal, and the receiver side applies 7-8 conversion to a received digital signal after demodulation and then applies error correction decoding to the 7-8 converted signal. Furthermore, in the case of 8-7 conversion and the 7-8 conversion as above, the transmitter side does not convert a signal of 1-series 8-bit outputted from transmission digital processing units TPDU 11, 21 directly into a signal of 1-series 7-bit but applies 8-1 conversion to the 8-bit signal after 1-7 conversion and the receiver side conducts 7-1 conversion after 8-1 conversion.