PURPOSE: To permit the measurement of flatness under a brighter condition than conventional measuring devices only by decreasing the amount of light by half of a source of light when measuring the flatness of glass plate etc. by applying to a Fizeau's interferometer.

CONSTITUTION: The polarized laser light 1 is emitted and composed a parallel beam by a collimator lens 3 and projected on a polarized beam splitter 4. Half of the amount of the incident light is penetrated to P wave component side at the splitter 4. The remaining half of the amount of light is bent to S wave component side at a right angle and penetrated a quarter wave length plate 5 and an optical flat 6 to generate an interference light. The interference light is reflected and penetrated the optical flat 6 side and repenetrated the quarter wave length plate 5 to penetrate the polarized beam splitter 4. The beam is focused into an image at a focal surface of a collimator lens 8 and the interference light is observed by an ocular 9. The amount of light at the time of measurement is only reduced to half of a source of light because the quarter wave length plate is provided at the S wave side of the splitter.