A luminous flux including laser light of different wavelengths outgoing from a light source unit is split into two luminous fluxes, the first luminous flux is focused on a sample with an objective lens, and the second luminous flux functions as reference light without radiating it onto the sample. Signal light reflected from the sample and the reference light are multiplexed by a polarized beam splitter and are made to interfere on four photodetectors out of phase in a photodetection unit. A signal processing unit acquires the optical axis distribution of an object in the sample by using the outputs of the plural photodetectors for every input wavelength, acquiring a detection signal and calculating the ratio of intensities of the detection signals at the different input wavelengths for every position in the sample.