PURPOSE: To improve the output frequency stability of a frequency modulator, by making variable the input control signal of a variable control oscillator in accordance with the output signal of a third phase deciding part.

CONSTITUTION: According the fact whether the oscillating output frequencies of a variable control oscillator 30 are higher or lower compared with the reference oscillating frequencies of a reference oscillator 31, it is determined whether a first phase difference deciding part 32 first decides the delay or advance of the phase between both the signals or a second phase difference deciding part 33 first decides its delay or advance. By deciding the delay or advance of the phase between the output signals of the first and second phase difference deciding parts 32 and 33 with a third phase difference deciding part 34, for example, when the output frequencies of the variable control oscillator 30 are higher than the reference oscillating frequencies, '0' can be obtained as the output signal of the third phase difference deciding part 34, end when lower, '1' can be obtained, and the variable control oscillator 30 is controlled in accordance with the output signal. Thus, the output frequencies of the variable oscillator 30 can be stabilized.