PURPOSE: To adjust the optical axis of the objective lens into an excellent state for the laser beam of a measurement optical system in a short time.

CONSTITUTION: A test plate which has a grating pattern PT is arranged at the focus position of the objective lens and the measuring optical system is provided with an eyepiece for observing the grating pattern. A rotating and moving mechanism rotates the objective lens on the optical axis and then moves the objective lens in an X or Y direction perpendicular to the optical axis while a grating pattern PT" rotating along an eccentric circle θ1 as the objective lens rotates is observed through the eyepiece, and the objective lens is stopped (a) at a position the eccentric circle θ1 becomes a smaller eccentric circle θ2. The optical axis of the objective lens is swung and tilted around the X or Y axis for the eccentric circle θ2 to make the eccentric circle θ2 smaller. The parallel movement and swing and tilt adjustment are repeated to make the eccentric circle approximate 0 and thus the optical axis of the objective lens is aligned with the laser beam (c), (c).