Electronic musical instrument having a portamento function utilizing whole and half transformations

A keyboard and a portamento bar are connected to a tone generator through a common channel assigner to use each tone generating channel either for generating a keyboard sound or for generating a portamento sound. Thus, a portamento sound can be generated without a need for providing a another tone generator. Further, such arrangement is provided that can shift the tone generation from a keyboard sound to a portamento sound and from a portamento sound to a keyboard sound, enabling initiation and termination of a portamento performance to have accurate pitches.