PURPOSE: To improve measuring accuracy of the titled instrument by detecting the quantity of light of laser beams passing through a pinhole at the prescribed position and performing alignment between the pinhole and the laser beams by a feedback mechanism based on the detected value.

CONSTITUTION: A part of the irradiated 1 laser beams is directed to the surface (S) of an object to be detected passing through a half mirror 6 via a diverging lens 2 and the pinhole 3 and a part of the rest is reflected on the mirror 6 and irradiated on a photodetection plate 9 and photodetected with detectors 10 (10aW10d) and converted into electrical signals in accordance with the quantities of photodetection. When these signals are compared respectively and the alignment between the pinhole 3 and an optical axis of the laser beams is adjusted and the alignment is made in an accurate state, interference fringes are formed on the laser beams via a collimator lens 4 interfering the reflected light of the surface (S) with the reflected light of a reference plate 5 and the interference fringes are reflected via the mirror 6 and received by a TV camera 8 and an image analysis is performed, by which a surface state of plane accuracy of the surface (S) can be measured.