Abstract: A digital DQPSK modulator according to the present: invention limits respective bands of symbol mapping data of I phase and Q phase obtained by applying differential encoding and mapping processings to a digital baseband signal, and multiplies the obtained data by the carrier signals by means of the digital filter. Such limiting of the bands and multiplication by the carrier signals are carried out by the digital filter in a time-divisionally multiplexed manner. The digital filter includes a circuit, for accumulating symbol mapping data for each of the I phase and the Q phase corresponding to a plurality of symbol sections, a plurality of ROMs corresponding to the plurality of symbol sections for storing a multiplication result of symbol data corresponding to a predetermined filter waveform and the carrier signal, and an adder for adding data output from these ROMs. The adder outputs a digital modulated signal which is, in turn, converted into an analog modulated signal by a D/A converter.