An object is to reduce the effect of a film thickness variation on the substrate surface of a thin film and improve the measuring accuracy. Provided are a light source that radiates single-wavelength light to an inspection-target substrate (W), which is formed by forming a thin film on a glass substrate from the glass substrate side; a light receiving element that is disposed such that the light receiving axis intersects with the optical axis of illumination light emitted from the light source at a predetermined inclination angle and that receives diffused transmitted light that has been transmitted through the inspection-target substrate W; and a computer (7) that obtains a haze ratio of the thin film on the basis of the intensity of the light received by the light receiving element. The computer (7) has a haze ratio characteristic made by associating the haze ratio and the light intensity of the diffused transmitted light and obtains a haze ratio by using the haze ratio characteristic and the light intensity received by the light receiving element.