A transmission data sequence including a plurality of continuous data portions each formed on a byte unit basis and including predetermined code data is transmitted from a transmission station to a receiving station as serial data. At the receiving station, the transmission data sequence is received and converted to parallel data, and the parallel data is processed byte by byte. At the transmission station, out-of-synchronization detection code data sequence derived by shifting the first predetermined code data by from one bit to a predetermined number of bits at one bit pitch is inserted into the transmission data sequence and it is transmitted. At the receiving station, when a part of the out-of-synchronization detection code data sequence in the transmission data sequence is detected during processing on a byte unit basis of the transmission data sequence received and converted to the parallel data, it is determined that the processing on a byte unit basis of the transmission data sequence is in the out-of-synchronization state between the transmission station and the receiving station and an operation to recover the processing on a byte unit basis of the transmission data sequence to a synchronization state is conducted.