To increase the speed of illumination pattern switching, this structured illumination device is provided with: an optical modulator (3) disposed within luminous flux output by a light source (11) and having a sound wave propagation path in a transverse direction to that output luminous flux; a drive unit (19) that gives rise to standing waves for sound waves within the sound wave propagation path by applying a drive signal for vibrating a sound wave propagation path medium to the optical modulator; illumination light systems (4, 6, 7, 9) that make diffraction components that differ from each other in the output luminous flux passing through the sound wave propagation path interfere, and form the interference fringes thereof on an article to be observed (10); and adjustment units (90, 19) that adjust parameters for the drive unit such that the state of the interference fringes is a prescribed state.
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