Abstract: A low coherence light beam is split into first and second light beams, and the frequency of the first light beam is shifted. The first and second light beams are then combined with each other at a position at which the optical path difference between the two light beams is larger than the coherence length of the low coherence light beam. The combined light beam is guided to a position in the vicinity of a medium having light scattering properties and split into a third light beam, which travels reversely to the direction of travel of the combined light beam, and a fourth light beam, which is irradiated to the medium. A light beam scattered backwardly from a predetermined depth in the medium is caused to interfere with the third light beam, and the intensity of the resulting interference light beam is detected.