A frequency modulating circuit (10) for modulating a carrier signal by a modulating signal (S,) is disclosed which includes a band-pass filter (10A) of biquad type having a first integrating circuit (12A) and a second integrating circuit (128), an output of the first integrating circuit (12A) being supplied to an input terminal of the second integrating circuit (12B) and an output of the second integrating circuit (12B) being supplied to an input terminal of the first integrating circuit (12A) and the input terminal of the second integrating circuit (12B), and a limiting amplifier circuit (10B) supplied with a part of the output signal of the band-pass filter (10A) and the output thereof being positively fed back to the band-pass filter (10A). In this case, the modulating signal (S l ) is supplied to the first and second integrating circuits (12A, 12B) such that the center frequency of the band-pass filter (10A) is linearly controlled by the modulating signal (S,) whereby a frequency modulated signal (Sm) modulated by the modulating signal (S,) is led out of the output terminal (11) of the second integrating circuit (12B).