An observation apparatus (100) includes an observing optical system (101) capable of obtaining an image of a measurement target present in a gap included in a device (1). One end of the gap included in the device (1) is wider than the other end thereof, and upon light beam irradiation to the device (1), an interference fringe appears in the gap. The observing optical system (101) irradiates the gap included in the device (1) with a plurality of light beams having different wavelengths to cause a plurality of interference fringes to appear in the gap. Then the observing optical system (101) obtains an image of the plurality of interference fringes.