PURPOSE: To eliminate the need for an automatic gain control amplifier, to minimize the change of a demodulating output, to widen a dynamic range and to hasten an action speed by amplifying logarithmically an intermediate frequency output, removing a direct current component, converting this reversely logarithmically and demodulating it.

CONSTITUTION: An intermediate frequency signal amplified from an intermediate frequency filter 7 is connected to an intermediate frequency part 8 as an input signal Vin. An output signal Vout', in which the logarithm is approximate and the envelope is detected from the intermediate frequency part 8, is connected to one side terminal of a capacitor 9, and the direct current component is cut. From other terminal of the capacitor 9, a signal Vout is connected to the input of a reverse logarithm conversion circuit 10 and converted reversely logarithmically to the signal Vout. From the output of the reverse logarithm conversion circuit 10, an output signal VAM' is connected to a speaker 13 through a band pass-filter 11 and an audible amplifier 12. Thus, a stable amplitude modulating wave signal can be demodulated, the automatic gain control amplifier is not necessary, the dynamic range can be widened, the action speed can be hastened and the integrating circuit of the intermediate frequency amplifier can be facilitated.