Abstract: In order to measure an inner defect of a sample with a certain high accuracy even if the sample surface of the moved up and down by flatness irregularity of the sample and problem on accuracy of the sample movement stage, incident light beams having two wavelength and respective different penetration depths for the sample are slantingly irradiated on the surface of the moving sample 15 from irradiation optical systems 4, 8, and the inner defect of the sample is measured by detecting the scattering light occurred from the interior of the sample with a detection optical system 9 arranged over the sample surface. A distance measurement means 14 is located in an upstream of a movement direction of said sample than said irradiation optical system 4, 8 and said detection optical system 9, thereby a surface height of said sample is measured.