Abstract: A quadrature phase modulator constructed by use of a simple digital circuit. Digital signals of sample data obtained by sampling outputs from a roll-off filter with a frequency four times a modulated carrier frequency are stored in an ROM, the outputs corresponding to all combinations of a plurality of successive bits of input data; the plurality of bits are set as an address, and a signal of an in-phase channel and a signal of a quadrature phase channel is alternately read from the ROM with a frequency equal to half the sampling frequency, then the obtained serial signals are subjected to a digital-to-analog conversion to obtain a quadrature phase modulated signal.