This method includes: an emission step for emitting polarized light onto a surface to be inspected; a light reception step for using two or more light reception means to receive, at two or more different light detection angles, reflected light resulting from the reflection of the emitted polarized light by the surface to be inspected; an image signal acquisition step for acquiring an image signal representing an image of the surface to be inspected from the reflected light received by each light reception means; a representative value calculation step for calculating representative values from the image signals for each light reception means; and a determination step for determining whether the surface to be inspected has a chemically converted film on the basis of the representative values calculated for each light reception means. In the representative value calculation step, if a gain adjustment step is not carried out between the image signal acquisition step and representative value calculation step, the representative values are determined from the intensities of the image signals, and if a gain adjustment step is carried out between the image signal acquisition step and representative value calculation step, the representative values are determined from the gains of the image signals.
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