PURPOSE: To make it possible to measure a minute interval, by forming a thin film comprising a metal oxide and the like, which control the quantity of reflected light, on a reference plane, forming a virtual reference surface comprising a light transmitting material, which is overlapped on the above described thin film, and contrasting the interference of the reflected light from the reference plane and reflected light beams from the virtual reference plane and the plane of a material to be measured.

CONSTITUTION: On a reference plane 11 of a dummy disk 1, which faces a plane 12 of a material to be measured, a metal oxide film 2 is provided. A quartz film 3 is overlapped on the film 2, and a plane 20 of a virtual film is formed. Light beams 10 reach the dummy disk 1 through a mirror 5 from a light source 6. Then, a clear reference- plane reflected light beam 12 is yielded by the metal oxide film 2. A faint metal-oxide- film reflected light beam 23 is yielded. A clear virtual-reference-film reflected light beam 24 is yielded. A clear reflected light 22 is yielded from the plane 12 of the material to be measured. Based on the interference phenomenon of the three kinds of the reflected light beams 21, 24, and 22, the distance t1+t2 between the reference plane 11 and the virtual reference plane 20 and the distance ST between the plane 12 of the material to be measured and the reference plane 11 are obtained. ST-(t1+t2) is computed, and the minute distance S can be measured.