An input waveform signal representing, for example, a string oscillation is input to a pitch extraction circuit. The pitch extraction circuit extracts a pitch frequency from the input waveform signal, and this pitch frequency is input to a RAM connected to a CPU. On the other hand, LFO data from an LFO is input to a RAM. The CPU detects the amount of variation of the pitch frequency based on the pitch frequency and coverts the amount of variation of the pitch frequency in accordance with a predetermined conversion function. Thereafter the converted value is added to the LFO data to form musical sound control data for imparting a tremolo effect or a vibrato effect. The musical sound production circuit thereby imparts the above effect to the musical sound to be produced.