PURPOSE: To realize a large amount of delay in small chip dimensions by adopting a plurality of FETs having a large gate width and a large gate length and being connected in series for a delay device.

CONSTITUTION: An input frequency modulation signal Vin=cos{2πt(10GHz+Δfo)} is divided in two signals, Vin1 and Vin2, by a distributor 1, and after the phase of the Vin1 is changed at 90 degrees by a phase shifter 2, this signal is inputted to a first input terminal 4 of a phase detector 3. Meanwhile, the Vin2 is delayed to τ=2000PS by a delay device constructed of FETs 8 of a width 300μm and a length 3μm and capacitors 9 connected in series, and then it is inputted to a second input terminal 6 of the phase detector 3. On the occasion, Vout=4000πΔfo (pV/Hz) at which the condition of fo>>1/τ is established is obtained. Since an active element is adopted, the loss of the delay device becomes zero and an amplifier is dispensed with. Accordingly, a frequency discriminator having high sensitivity and a wide band and being small in size and suitable for making a construction monolithic is obtained.