An interferometer measures a displacement of an object to be measured by observing a fluctuation in intensity of interfering light generated by dividing light emitted from a light source (LD) into two light beams and overlaying the two light beams. The interferometer includes: a light-receiving unit (PDC) including a light-receiving area including a plurality of partial areas (PDC1-PDC4) and configured to detect the interfering light in each of the plurality of partial areas; and a processing unit (RVR) configured to calculate a value of an index indicating uniformity of a phase distribution of the interfering light in the light-receiving area by using a detection result in each of the partial areas.
