A first light beam and a second light beam having discrete wavelength bands are emitted form a light source unit, and enter a light divider. The light divider separates each light beam into a measuring light beam and a reference light beam. The measuring light beams are irradiated on a measurement target, and reflected light beams, which are reflected at various depth positions of the measurement target, are caused to enter a combiner. The reference light beams propagate through optical fibers to enter the combiner. Interference light beams formed by the reflected light beams and the reference light beams for each of the first and second light beams are photoelectrically converted into interference signals. A tomographic image is obtained employing the interference signals for each of the first and second light beams.