A detector for a modulated AM stereophonic signal of simple configuration in which the sub signal of the stereophonic signal is detected accurately independent of fluctuations in the frequency of the carrier signal. In the detector circuit, the output phase of a VCO in a PLL circuit which is fed with an amplitude limited signal corresponding to the output of an IF amplifier in the receiver is controlled by a variable phase shifter that performs phase shifting according to a control voltage. The phase-controlled output is used as a synchronous detection signal for sub signal detection. The control voltage is produced to correspond to the phase difference between the phase-controlled signal and the carrier signal of the received AM stereophonic signal.