PURPOSE: To adjust the angle and the position of incidence of a luminous flux easily and accurately and make a position sensor small-sized by providing independent reflecting planes in reflecting optical systems so that most of reflecting optical systems is directed to the same direction as coming of parallel rays of a luminous flux.

CONSTITUTION: Parallel rays of the luminous flux from a light source pass an ND filter 17 and a convex lens 3 and are divided into two by a beam splitter 4, and the light transmitted through the beam splitter 4 is led directly to an area sensor 5. Meanwhile, the light reflected on the beam splitter 4 is returned to the beam splitter 4 by the reflecting optical system consisting of a prism 15 and a plane mirror 6 and is transmitted through the beam splitter 4. This transmitted light is returned to the beam splitter 4 again by the reflecting opti cal system consisting of a prism 16 and a plane mirror 7 and is reflected on the beam splitter 4 and is led to the area sensor 5. Since optical systems are adjusted while seeing simultaneously the position and the angle of the luminous flux by the output of the area sensor, easy and accurate adjustment is possible.