Provided is a method for determining the optimal resonant length, among a large number of local resonant lengths which satisfy a resonant condition, that maximizes wave intensity in a resonant structure. In the second harmonic generation or the cascaded difference frequency generation device using the resonant structure by which the second harmonic wave of a pump wave resonates, the optimal resonant length is determined so that the intensity of an optical wave is maximized. The intensity distribution of the optical wave is defined, according to the resonant feedback condition or the no-resonant feedback condition, as a function of the resonant lengths to determine the optimal resonant length.