PURPOSE: To adjust an optical system esily and securely by detecting the optical adjustment state of a condenser lens electrically by using a beam splitter, pinhole mask, and photodetecting element.

CONSTITUTION: The optical system is provided with the photodetecting element which has the beam splitter 36 and pinhole mask 38 and shutters 42 and 44 for the preliminary adjustment of the optical system. When a light source 10 emits light while the shutters 42 and 44 are closed, information light from a converter lens 20 is split partially the beam splitter 36 into adjustment light, which is incident on the element 40 through the mask 38. When the adjustment light is converged on the position of the pinhole of the element 40, the adjustment light is all incident on the element 40, whose output is maximum. Thus, such an optical system arrangement that the measurement light and reference light are converged on mirror surfaces of plane mirrors 26 and 28 is realized. For the purpose, the position of the condenser lens is so adjusted that the output of the element 40 is maximum, thereby making an optical adjustment easily and securely.