PURPOSE: To measure the reflectance of even a lustrous material, by reflecting light from a light source by a reflecting mirror in a rotating-body shape, and making light which is reflected specularly by a sample surface after passing through the opening part of a photodetecting element incident to the photodetection surface of the photodetecting element.

CONSTITUTION: The reflecting mirror 6 is formed in a part of an ellipsoidal surface of revolution which has focuses in the centers of a light source 5 and photodetecting element 4. The light 5a from the light source 5 is reflected by the reflecting mirror 6 to illuminate the sample surface 8a through the opening part 4b formed in the center of the photodetecting element 4. The sample surface 8a is irradiated at some angle to its normal, so specularly reflected components of the reflected light are all incident to the photodetection surface 4a and almost all diffused components are made incident to the photodetection surface 4a. The photodetecting element outputs of this measuring instrument are measured with respect to a standard reflecting plate and the sample, and the reflectance of the sample is found from the output ratio.