Using an optical tomography method of splitting low coherent light into sample light and reference-light, emitting the sample light to a measurement-target in a line shape, generating interference light by superimposing reflected light from the measurement-target due to emission of the sample light and the reference-light on each other, and acquiring a two-dimensional spectroscopic tomographic-image of the measurement-target by spectroscopically detecting the interference light and performing frequency analysis, an arbitrary wavelength region in an ultraviolet region is cut out from low coherent light including a wavelength region from an ultraviolet region to a visible region and the arbitrary wavelength region is shaped into a spectrum having an arbitrary wavelength width, the two-dimensional spectroscopic tomographic-image is acquired as using the low coherent light, and the penetration depth of the sample light for the measurement-target is evaluated based on the two-dimensional spectroscopic tomographic-image.
