To provide a frequency discriminator for direct converting receiver with which circuit configuration is simplified by simplifying operation for FM demodulation by the method of direct conversion.

Two in-phase and quadrature signal components I and Q of base band provided by directly converting an FM signal through a local oscillator 2, a 90° phase shifter 3, multipliers 4 and 5 and LPF 6 and 7 and AMP 8 and 9 are amplified to a fixed amplitude and converted later to digital signals by A/D converters 10 and 11. These digital signals are applied to differentiators 12 and 13, differentiated (differential) values are obtained, these values are squared by squarers 14 and 15, respective squared values are added by an adder 16, the root square value is provided by applying the added output to a square rooter 17, and the demodulated output of the FM signal is provided. An ROM 18 can be used in place of the squarers and the rooter as well.