Abstract: For the purpose of sequentially varying a tone pitch of a generated musical tone, a variation information generator is provided which is operable by a performer to generate a variation information designating an arvitrary variation rate of the tone pitch. An operation circuit is provided for producing a modified frequency information in response to the variation information and a frequency information representing a tone pitch regarding a depressed key. The modified frequency information is supplied to a musical tone signal generator and the output thereof is applied to a sound system for generating the musical tone, thereby obtaining a musical effect similar to glissando or portamento and having an arbitrary variation rate.