PROBLEM TO BE SOLVED: To improve the accuracy of measurement by disposing a sheet of plate glass whose one end is of a wedge shape and the other a parallel form behind a collimeter lens, and moving for adjustment the lens so that an interference stripe on a screen is turned in a lateral direction.
SOLUTION: An optical system is composed of a laser beam source 1, microscope objective lens 2, collimeter lens 3, plate glass 4 whose one end is in a wedge form and the other in a parallel form, focusing lens 5, and screen 6. The plate glass 4 is coated with a metal reflection film on each of the opposed sides of the plate glass 4. The plate glass 4 is disposed behind the collimeter lens 3, and light made parallel is made incident to the glass plate 4. When the glass plate 4 is set slantly slightly to give a shear, a sharp interference stripe is focused on the screen 6 by a multiple beam, and when the beam is mad parallel completely the interference stripe generates laterally. Accordingly, through the adjustment of moving the lens 3 along an optical axis to turn the interference stripe in the lateral direction, the collimation of beam can be carried out.