A carrier wave having a sub-audible frequency is modulated in amplitude by a modulating wave of a tone signal having an audible frequency, thereby producing a resultant modulated output signal including side band components deviated from the original tone signal frequency by a deviation amount equal to the carrier frequency. The frequency of the carrier wave is selected to be much lower than that of the modulating wave. The frequencydeviated signal thus obtained is fed back and admixed with the modulating wave to obtain a multiplicity of frequency-deviated signals. These signals are mixed with the original tone signal to produce an intricate tremolo effect.