A first beam having high coherence and a second beam having low coherence are multiplexed onto the same optical axis. This multiplexed beam is split into first and second multiplexed beams. The first multiplexed beam is directed towards a measurement reflection plane of an object to be measured, and the second multiplexed beam is directed towards a reference plane. The first and second multiplexed beams that are reflected from the measurement reflection plane and the reference plane, respectively, are multiplexed to cause the first beams to interfere with each other and the second beams to interfere with each other. Calculations are carried out to determine a measurement origin on the basis of the first and second interference signals obtained from the first and second beams, respectively.