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 into two light beams and overlaying the two light beams. The interferometer includes: a light-receiving unit including a light-receiving area including a plurality of partial areas and configured to detect the interfering light in each of the plurality of partial areas; and a processing unit 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.