Reflectance measurement with two monochromatic light beams having different wavelengths is used to obtain curves respective representing the relationship between an insulation film thickness of a semiconductor wafer and the gap between a test electrode and a semiconductor wafer surface. The C-V curve measurement at a fixed gap determines a total capacity of the gap and the insulation film, and a straight line representing the relationship between the gap and the insulation film thickness is obtained from the total capacity. An intersection where the two curves and the straight line cross gives the true values of the gap and the insulation film thickness. Other possible methods include: one for executing the C-V curve measurement and the reflectance measurement with two linear polarized light beams having identical wavelengths but different polarization directions; one for executing the reflectance measurement with three monochromatic light beams that differ in at least wavelength and/or polarization direction; and one for executing the reflectance measurement and the C-V curve measurement for two different gaps.