A structured illumination device includes: a diffraction unit that diffracts light beams of a plurality of wavelengths that are emitted simultaneously or sequentially by a light source into a plurality of diffracted beams; and an optical system that forms interference fringes on a surface of a sample using the plurality of diffracted beams diffracted by the diffraction unit, the optical system including a first optical system and a second optical system that focuses the plurality of diffracted beams at positions on or near a pupil plane of the first optical system, and a magnification characteristic dY(λ) of the second optical system satisfying the condition of (fo·nw−afλ/P)≦dY(λ)≦(fo·NA−afλ/P), where a=1 (for M=1, 2) or a=2 (for M=3).