Only in a transient band of a filter, substantially all frequency bands of the musical tone waveform are subjected to filter processing. Due to this, the drawback that the amount of change of the frequency characteristic of the musical tone gradually changes as a whole from a fundamental wave toward a harmonics and only one part of the frequency characteristic of the musical tone changes is eliminated. Also, the density of frequency components of the frequency band of the musical tone waveform does not change, the related frequency band is shifted in frequency, subjected to filter processing, and further shifted to a frequency in accordance with the musical tone pitch. Due to this, a specific range of the filter characteristic is selected, and the musical tone is subjected to the filter control only within this range. Then, after the filter processing, the musical tone is shifted in frequency in accordance with the musical tone pitch, and therefore the musical tone is subjected to the filter processing irrespective of the musical tone pitch. Further, the density of the frequency components of the frequency band of the musical tone waveform does not change and the related frequency band is shifted in frequency. Due to this, a harmonics ratio of the frequency components of the frequency band changes, a timbre (musical tone quality) finely changes, and a control on the musical tone which has not conventionally existed is achieved. Also, gains of boundary portions of frequency bands of a plurality of partial musical tone waveforms subjected to the filter processing are matched. Due to this, the gains of the boundary portions of the frequency bands of the partial musical tone waveforms are matched, and a well balanced synthesized musical tone is output.