Light emitted from a light source (22) is used through a light projection optical system (23) to perform coaxial down-emission lighting on a measurement target (36). Light reflected by the measurement target (36) is formed on a photo-detector (26) through an image formation optical system (24). Along its optical path, a spectroscope (25) is provided for converting an image impinging on the photo-detector (26) into a spectroscopic image having a predetermined wavelength band. A measurement point extraction portion (32) in a signal processing portion (28) determines a predeterm film thickness measurement point from an image picked up by the photo-detector (26), extracts an image signal at the film thickness measurement point, and transmits t to film thickness operation portion (33). The film thickness operation portion (33) measured film thickness of a thin film, which is the measurement target (36), from this signal.