PURPOSE: To provide an FM signal digitizing circuit which can improve the processing precision of FM demodulation or the like by reducing a quantizing error contained in a digital FM signal.

CONSTITUTION: An analog FM signal is amplified by a variable gain amplifier 14 corresponding to an amplitude control signal, inputted to an A/D converter 3 after the output amplitude is limited by a limiter 16 and a higher harmonic wave component generated due to the amplitude limitation is removed by an LPF 17 later, and converted into the digital signal. Then, the amplitude level of the converted digital FM signal is detected by an amplitude detection circuit 15, the error concerning the detected amplitude level is calculated by calculating the difference from a specified level at a subtracter 11, and an amplitude control signal for correcting this error is generated by an integrator 12 and a D/A converter 13 and transmitted to the variable gain amplifier 14. Thus, the input amplitude of the FM signal to the A/D converter 3 is made constant, and the lack in gradation at the time of A/D conversion is eliminated.