A transmitter cyclic pattern having a pattern length of N bits is generated and converted into an M-bit transmitter parallel data stream, where N≠M. A bit-sequence altered transmitter parallel data stream is generated by performing a transmitter altering process, converted into a serial data and transmitted together with a clock signal. The serial data is received and converted into an M-bit receiver parallel data stream, and a bit-sequence restored parallel data stream is generated by performing a process opposite to the transmitter altering process. A receiver cyclic pattern is generated by using bits in the bit-sequence restored parallel data stream and converted into an M-bit reference parallel data stream, and a bit-sequence altered reference parallel data stream is generated by performing a process same as the transmitter altering process and compared with the received parallel data to test if the data is correctly received.