OBLIQUE INCIDENT INTERFEROMETER

PROBLEM TO BE SOLVED: To match paths of both a reference light and an object beam with a simple constitution by arranging a path matching means at one of a light separator and a light synthesizer and a diffraction grating at the other, constituting the means of two mirrors non-parallel with each other, and regulating a distance between the two mirrors to thereby correcting an optical path difference between the reference light and the object beam. SOLUTION: In order to generate a reference light 5 and an object beam 7 by separating a light having a short coherence length from a light source 1, a path matching means 30 having two half-mirrors 3, 4 is provided. The first half-mirror 3 reflects a part of the incident parallel beams toward a surface 6a to be detected and transmits the excess of the beams. The second half-mirror 4 reflects part of the beams transmitted through the mirror 3 toward a diffraction grating 8. The two mirrors 3, 4 are non-parallel with each other. A predetermined optical path difference is incorporated between the reflected lights from them by regulating their distance, and the path difference between the light 5 and the beam 7 is reduced in the state that they arrive at the grating 8.