An inspecting method inspects an optical stacked structure having a reflection layer and at least one light transmitting thin film sequentially stacked on a substrate. The inspecting method irradiates inspection light on the optical stacked structure from a side provided with the light transmitting thin film, measures a light intensity of reflected light from each layer, that changes depending on a change in an optical path length to each layer, and inspects a thickness of the light transmitting thin film based on the light intensity of reflected light for a specific wavelength.