A beam emitted from a light source is split into a probe beam that irradiates a measurement object and a reference beam that does not irradiate the measurement object. A signal beam obtained by the reflection of the probe beam is split into first and second split signal beams, which are mutually orthogonal polarized components. The first split signal beam and the reference beam are inputted to a first coherence optical system to cause the beams to interfere with each other to generate at least three coherence beams differing in phasic relationship. The second split signal beam and the reference beam are inputted to a second coherence optical system to cause the beams to interfere with each other to generate at least three coherence beams differing in phasic relationship. The coherence beams are then detected.