A musical tone signal is synthesized on the basis of a predetermined modulation operation (e.g. an FM or AM operation) employing a modulation signal and a carrier signal respectively having an audio range frequency. In a waveshape table provided for defining at least one of a modulation wave function or a carrier wave function, a wave function expressed in a logarithmic form log{f(x)} is stored. The wave function log{f(x)} read out from this waveshape table is multiplied with a coefficient k whereby the wave function of the modulation signal or the carrier signal to be used in the modulation operation is changed from f(x) to {f(x)}.sup.k. That is, k log{(f(x)}=log{f(x)}.sup.k is obtained by this multiplication and {f(x)}.sup.k is obtained by converting this log {f(x)}.sup.k to a linear form. Therefore, it is enabled to synthesize a tone having abundant frequency components by using the wave function {f(x)}.sup.k obtained by a simple operation.