PURPOSE: To shorten the control time together with omission of the control expertness by inputting the modulation signal to a frequency modulator through A/D converter, a ROM, a D/A converter and an LPF.

CONSTITUTION: An A/D converter 2, an ROM 3, a D/A converter 4, an LPF 5 and an amplifier 6 are connected in series to a modulation input terminal IN. Then an output terminal is connected to a modulation input terminal 1a of a frequency modulator 1. Thus the modulation output is outputted through a modulation output terminal OUT. In such a way, the modulation linearity of the frequency modulator is improved and the control of frequency is simplified by applying the digital processing to the modulation signal.