PURPOSE: To measure unevenness of thickness by projecting a laser beam on to a transparent body, installing a means for leading reflected light beams of front and rear surfaces to a light detector and a beam for shifting the irradiating position and by counting output fluctuation waves of the light detector.

CONSTITUTION: Beams from a laser source 1 are oriented parallel by a collimator lens 2 and into narrow beams by a slit 3, and then they are reflected on front and rear surfaces of an optical disk substrate 11 representing a transparent body and an image is developed on a light detector 7. At this time, interference is caused between the reflected beams from front and rear surfaces and in accompaniment of thickness of the substrate 11, these beams are strengthened or weakened. These results are taken out by the light detector 7 as a fluctuation signal and by counting number of fluctuation waves, unevenness of thickness of above the wave length order (below 1μ) can be measured.