To provide an inspecting method capable of readily inspecting a film of a surface of a wafer with high sensitivity.

The inspecting method employs an inspecting device that includes: a stage for supporting a wafer having a film provided on its surface; a lighting system for projecting illumination light to the surface of the wafer supported by the stage; a detector for detecting reflection light from the surface of the wafer to which the illumination is projected; and an inspection unit for inspecting the film, based on information about the reflection light detected by the detector. The inspecting method includes a setting step S101 of setting a target film thickness and a refraction index, a computation step S102 of computing a characteristic of change in the refraction index with respect to change in the film thickness in the vicinity of the target film thickness in accordance with the target film thickness and the refraction index by each of a plurality of wavelengths, and a determination step S103 of determining that a wavelength in which the change in the refraction index (i.e., sensitivity) becomes maximum is made to be the wavelength of the illumination light on the basis of the characteristic of the change in the refraction index with respect to change in the film thickness.