Abstract: Compensation is performed to eliminate noise generated by fluctuation of a medium that surrounds an object to be measured. First and second laser beams having different wavelengths are generated by a He--Ne laser and a YAG laser, respectively, and are superposed by a first dichroic mirror such that they propagate along a common path. The first laser beam is caused to reflect off the object to be measured, while the second laser beam is reflected by a second dichroic mirror before reaching the object. The first and second reflected laser beams are separated from each other by a third dichroic mirror. The reflected first laser beam is detected by a first position sensor, while the reflected second laser beam is detected by a second position sensor. This makes it possible to compensate for noise generated by fluctuation of the medium around the object to be measured, so that the object can be measured accurately and reliably.