Observation apparatus

An observation apparatus comprising:

• a structured illumination grid and light source that emits structured illumination light having a light-dark pattern;
• a phase difference measurement illumination slit and objective lens that emits illumination light for phase difference measurement onto a subject-mount surface;
• 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 having passed through the subject-mount surface is incident into the phase contrast lens, and the structured illumination light is incident into the phase contrast lens in a direction different from a direction of incidence of the illumination light for phase difference measurement through an optical system;
• a sensor that detects reflected light of the structured illumination light which is reflected on the subject-mount surface;
• and
• a set of an objective lens and sensor that images the illumination light for phase difference measurement which has passed through the phase contrast lens,
• wherein a spatial frequency of the structured illumination light is set such that a position of an image of the structured illumination light applied Fourier transform is positioned on a high-frequency side or a low-frequency side with respect to a position of the phase plate on optical Fourier space, by adjusting a pattern of the grid.