PURPOSE: To easily obtain detection characteristics equal to that of a circuit which has turning characteristics with a high Q value, by decreasing the ratio of An FM and an AM noise having a certain relation below a specific value.

CONSTITUTION: The frequency shift Δω of an FM noise contained in a received signal and the amplitude of an AM noise have a relation Δi=F(Δω). When slope detection characteristics are expressed by f(i,ω), the gradient of slope characteristics is df/dω and the detected noise current of the FM noise is (df/dω)Δω. Therefore, a noise detection current is the sum df/di.F(Δω)+df/dω.Δω of both noise detection currents. For this purpose, the df/dω and df/di are so set that the current value is a half as great as or less than the half of the amplitude of the AM noise detection current. Consequently, an expression holds. Thus, detection characteristics equal to that of a circuit having tuning characteristics with an extremely high Q value are obtained easily and the FM noise and AM noise having a constant relation mutually cancel.