PURPOSE: To make it completely unnecessary to use a clock signal for restoration processing by executing conversion processing by a converter provided with (n) signal detecting signals and restoring the converted signal into the original serial signal by a converter provided with a circuit for outputting the exclusive OR of all n-bit parallel signals inputted to the circuit.

CONSTITUTION: Only when an input signal Din transits its state at the timing of each 2 bits from its initial one bit, a signal P1 also transits its own state with 2-bit delay. On the other hand, a signal P2 transits its own state with 2-bit delay only when the input signal Din transits its own state at the timing of each 2 bits from the 2nd one bit. A serial/parallel conversion circuit 1 converts the inputted serial signal Din into 2-bit parallel signals P1, P2 and outputs respective signals to transmission lines 2a, 2b. On the other hand, a parallel/serial conversion circuit 3 on the receiving side is constituted of only one exclusive OR circuit X10 connecting the transmission lines 2a, 2b to its input terminals.