A musical tone synthesizing apparatus comprising FM operators and connection control device. At least one of the FM operators comprises a waveform memory, phase data generation device, and output inversion device. The waveform memory stores fundamental waveform data. The phase data generation device generates phase data as an address signal for accessing the waveform memory on the basis of a musical tone designation signal and a modulation signal inputted from an external circuit. The output inversion device selects whether a waveform signal read out from the waveform memory is outputted after sign inversion, or is outputted directly. The connection control device arbitrarily combine connections of the FM operators.