In an interferometer for detecting interference light between light flux passed through an object to be inspected and reference light to be generated from a portion of the light flux passed therethrough, a phase of the interference light is detected with high precision. The light flux passed through the optical system to be inspected forms a spot image on a pinhole formed in a plate. Measuring light from the spot image and the reference-light diffracted out of the light flux from the spot image at the pinhole create interference light which in turn is received by an observation system. An image of interference fringes formed by the interference light is taken with an image pickup element. Further, heterodyne interference light is created by vibrating the plate in the direction intersecting the light flux or in the direction along the light path of the light flux, thereby detecting a phase of each portion of the interference fringes with high precision.