PURPOSE: To measure the regularly reflected component and diffused component separately by shielding part of the light flux for irradiation of a specimen, installing a mask as intercepting and retreating from the irradiative optical path and reflective optical path in a microscope, and thereby blocking the part corresponding to the regularly reflected light flux from the portion which was not shielded.

CONSTITUTION: A mask 1 shields the B-part and a-part by its semicircular photo- transmissive part, while a mask 1' blocks the A-part and b--part when it is placed on the optical axis of a microscope. When the photo-transmissive part 5 for light flux restriction of a disc 4 lies on the optical axis, the measuring value T is formed as summarization of the sum of the regularly reflected portion Sa of the light flux A-part and the random reflected portion Db of B-part and the sum of the regularly reflected portion Sb of B-part and the random reflected portion Da of A-part. Therein the condition T=Sa+Db+Sb+Da is met, and the measuring value when the mask 1 lies on the optical axis is the random reflected portion Da of A-part. In the same manner, Db is determined while the mask 1' is laid on the optical axis. Accordingly the measuring value To of the regularly reflected light due to reflected light from the specimen S and not including random reflection can be determined from To=T-Da- Db.