A musical tone forming device comprises circuit system of digital calculation realizing frequency modulation technology in three different modes. In the first mode a plurality of frequency modulated waveforms are individually calculated in response to key depression and resultant data is added together to provide a musical tone signal, in the second mode, a signal waveform having a frequency corresponding to a depressed key is frequency-modulated with a plurality of signal waveforms, and in the third mode a signal waveform of a frequency corresponding to a depressed key is frequency-modulated in a multiple manner with a plurality of signal waveforms. Throughout these systems, the same component parts are commonly used and tone signals of the respective systems can be produced by a simple switching operation.