A first beam having high coherence and a second beam having low coherence and having a central wavelength difference from that of the first beam are multiplexed onto the same optical axis. First and second multiplexed beams obtained by beam splitting are emitted at a measurement reflection plane and a reference plane, respectively. The reflected first and second multiplexed beams are multiplexed and interfere with each other. The interference generates a first interference signal that is obtained from the first beams at the interference unit and that relates to information on the distance to the measurement reflection plane and a second interference signal that is obtained from the second beams. The first and second interference signals are used to carry out calculations for determining the position of a measurement origin for the measurement reflection plane.