To generate an approximate waveform with a simple structure at low cost in the generation of an optional function waveform and the electro-optical use of the function waveform.

This waveform composition method for composing a waveform g(t) which approximates a waveform n(t) that is the function of a desired time t comprises steps of determining frequencies f1-fm of component waveforms constituting the waveform g(t); setting one or more prescribed time sections; determining the respective amplitudes and phases of the component waveforms so that the value obtained by integrating the square of the difference between the waveform n(t) and the total sum of the component waveforms over a time section; and giving the waveform g(t) as the sum of the component waveforms having the determined amplitudes and phases.