An observation device includes: a structured illumination section; a phase difference measurement illumination section; a phase contrast lens that has a phase plate for dimming illumination light for phase difference measurement, where the illumination light for phase difference measurement is incident into the lens, and the structured illumination light is incident into the lens from a side opposite to an incidence side of the ring-shaped illumination; a detection section that detects reflected light of the structured illumination light; and an observation section that images the illumination light for phase difference measurement. When Fourier transform is performed on the structured illumination light, a spatial frequency of the structured illumination light is set on a high-frequency side or a low-frequency side with respect to a position of the phase plate on optical Fourier space.