A voice keying system for a voice controlled musical instrument comprising a frequency responding circuit for responding to an input signal of audio frequency and generating a control signal which corresponds to each of a plurality of frequency bands of input signal and also to a plurality of notes of a musical scale, and a tone generator for generating an output tone signal corresponding to each of the notes of the musical scale. The input signal is converted into the output tone signal having a smaller rate of frequency increment than that of the input signal in each of the frequency bands thereof.