JP2949847B

PURPOSE:To enhance measuring accuracy by eliminating the effect due to the fluctuation of a light source by arranging a reference mirror so as to equalize the respective optical lengths from the surface of an object to be measured and the reference mirror to a detection part. CONSTITUTION:The change-over of a highly accurate optical system and a simple optical system is made possible and, in the simple optical system, the length of the light path from the surface 18 on an object to be measured to an optical sensor 62 and that of the light path from a reference mirror 42 to the optical sensor 62 both of which are the lengths of the light paths of measuring beam Lp and reference beam Ls obtained by splitting baser beam L by a polarizing beam splitter 40 are made equal. By this constitution, since the optical lengths of the beams Lp, Ls simultaneously emitted from a light source coincide each other, the fluctuation of frequency due to the disturbance of a light source 30 is cancelled when laser beams allowed to interfere each other in a light detection part and an error can be reduced without receiving the effect of disturbance to enhance measuring accuracy.