Abstract: The present invention relates to an optical interference measuring apparatus and method for measuring displacement of an object. The apparatus and method reduces the displacement-measurement error caused by changes of the refractive index of the atmosphere or the like, in a measurement optical path and a reference optical path, by suppressing crosstalk caused by unfavorable mixing of reference light and measurement light with each other, thereby improving accuracy in the measurement. The apparatus produces interference between a measurement light beam and a reference light beam. The measurement light is propagated through a measurement optical path including a measurement reflector, which is at least movable along the measurement optical path. The reference light is propagated through a reference optical path including a reference reflector. The interference occurs in a predetermined interference system, whereby the displacement of the measurement reflector can be measured.