PURPOSE: To enable measurement of a surface reflection of an object to be measured such as transparent glass without requiring a pretreatment thereof, by shielding reflected light from the back of an object to be measured structurally to make reflected light alone from the surface thereof incident into a detector.

CONSTITUTION: Light irradiated from a light source 1 is focused with a concave mirror 2 to irradiate the surface of a sample 6 with an imaging lens 4 through a fine clearance 3. The sample 6 is set at an opening of an integrating sphere 5 slightly inclined to the optical axis of an irradiation light and the irradiation light is introduced at the other opening thereof. The integrating sphere 5 is provided with a window to admit light into a photodetector 7. The irradiation light is reflected partially from the surface A of the sample 6 while being transmitted partially therethrough and further reflected from the back B thereof while the rest thereof passes through the back face thereof. When an imaging point of the irradiation light is set near the point C at which the back reflected light passes through the surface of the sample, an interval in the luminous flux outer diameter between the surface reflected light and the back reflected light is the greatest near the C of the surface. Thus, a blade tip of a shielding plate 8 is placed near the point C to shield the back reflected light, thereby enabling separation and measurement of the surface reflection of the object being measured from the back reflection thereof.