PURPOSE: To attain sufficient reproducibility by adding a high frequency signal having a sufficiently higher frequency than that of a modulation signal, and adding a DC bias and the modulation signal to a varactor diode being a component of a frequency modulator so as to improve the linearity of the frequency modulator comparatively easily.

CONSTITUTION: A high frequency signal is fed to an amplitude control section 41 comprising pin diode 411, 412 connected in opposite polarity via a buffer amplifier. Since a modulation signal whose phase is inverted through an inverter 421 and a variable attenuator 422 and which is converted to a proper amplitude is fed to the section 41, the resistance of the diode is maximum when the amplitude of the modulation signal is negatively maximum and the resistance is minimum when the amplitude is positively maximum. Then the amplitude of the high frequency signal is minimum when the amplitude of the modulation signal is negatively maximum and maximum when the amplitude of the modulation signal is positively maximum and the result is fed to a synthesis circuit. The oscillation frequency is changed depending on the amplitude of the harmonics of the higher frequency signal.