A modulation and demodulation system capable of minimizing a bit error rate in a six-phase phase modulation method. A senary signal phase-modulated and outputted by a modulator is received and phase-modulated by a destination demodulator to a binary signal before conversion by the modulator. The modulator assigns (0, 0), (0, 1), (0, 2), (1, 2), (1, 1) and (1, 0) which are senary signals (bi, ti) to first to sixth phases respectively. The demodulator performs a conversion process from the senary signals to the binary signals, for instance, by storing transmitted senary signals and sequentially converting every senary signal of length m to binary signal of length b so as to output them. The process of the demodulator assigns the first to sixth phases as the senary signals (bi, ti) to (0, 0), (0, 1), (0, 2), (1, 2), (1, 1) and (1, 0) respectively.