To easily generate a digital modulation signal which suppresses the occurrence of unstable waveform.

The digital modulator includes a coefficient ROM 1 for storing two kinds of coefficients related to a first modulation frequency f1 and two kinds of coefficients related to a second modulation frequency f2; a cumulative addition operating portion 2; and a CORDIC 3. The cumulative addition operating portion 2 selects the coefficients related to either frequency according to the values of modulation data. Out of two kinds of coefficients related to the selected frequency, the cumulative addition operating portion 2 uses one of the coefficients when the cumulatively added value does not exceed a threshold while using the other coefficient when the cumulatively added value exceeds the threshold, and adds up the coefficient values successively at each sampling frequency. The CORDIC 3 calculates the amplitude of a sinusoidal wave corresponding to an angular frequency at each sampling point calculated by the cumulative addition operating portion 2. Without using a PLL circuit which requires a predetermined convergence time for changing the frequency, an FSK modulated signal can be generated by digital calculation using only a small number of coefficients.