Abstract: An electronic musical instrument having a number of keys, having tone generators capable of simultaneous tone production, the tone generators being smaller in number than the number of keys. The instrument forms an operation for synthesizing a desired waveform, the operation for synthesizing a desired waveform being performed in a repeating cyclic order with an operation cycle and in which the waveform is transferred to the tone generators and read out therefrom at a rate in accordance with the note of a key being depressed to obtain a desired musical waveform. The device includes a number-of-depressed keys detecting device which counts the number of keys which are actuating the tone generator upon depression. A cycle altering device is provided for changing the operation cycle, as a whole, on the basis of the number of depressed keys, counted by the number-of-depressed keys detecting device. The construction allows for a waveform of a smooth temporal variation to be produced.