PURPOSE: To obtain a signal discriminator which never mistakes the monaural mode for the sound multiplex mode by detecting the presence or absence of a subcarrier to suppress the operation of a subcarrier detecting means or a pilot signal frequency selecting means.

CONSTITUTION: Since the subcarrier component appearing at an SIF detector 2 keep a prescribed level in receiving the normal added carrier, an output phase- comparing 19 the carrier and an output of a 1/2 frequency divider 16 keeps a prescribed high level. A muting circuit 20 discriminates the amplitude of this high level output and applies a signal to an AGC circuit 5. The AGC circuit 5 receives this signal to operate normally an AM detector 5, and applies a pilot signal to a discriminating circuit 7, where the sound multiplex mode is discriminated. When a television signal without the added carrier is received, an output of the SIF detector 2 goes to a low level, resulting that the signal representing it is fed to the AGC circuit 5 and the AM detector is controlled to inoperative state.