A digital modulating/demodulating apparatus includes a demodulator for demodulating a modulated signal into a demodulated positional signal in response to a receipt of the modulated signal, a decoder for decoding the demodulated positional signal into a digital signal of N bits. A previous signal point on the constellation plane is defined by a previous demodulated positional signal and a current signal point on the constellation plane is defined by a current demodulated positional signal. The decoder outputs a digital signal of N bits, based on en amplitude and a phase of the previous signal point and the amplitude and the phase of the current signal point.