PURPOSE: To obtain a highly sensitive demodulating circuit of a light FSK signal by fixedly weighting passing characteristics of plural signal passages corresponding to a demodulation frequency in a frequency discriminator in proportion to the S/N of a signal that pass the passage.

CONSTITUTION: A gain adjusting circuit 21 that makes fixed weighting in proportion to signal to the noise ratio (S/N) of signals that pass a branch part 4-2 is provided. The ratio K of output noise intensity N2 of the gain adjusting circuit 21 to output noise intensity N, of a branch part 4-1 set to a fixed value K=(S2/N2)/(S1/N1) when the S/N of a signal S2 that passes the branch part 4-2 to which the circuit is inserted is S2/N2 and the S/N of signal S1 that passes another branch part 4-1 is S1/N1. Thereby, the S/N can be made to a maximum, and a highly sensitized light FSK signal demodulating circuit can be realized.