PURPOSE: To detect positional deviation and slope between a main reflection mirror and a sub-reflection mirror.

CONSTITUTION: This device is provided with the main reflection mirror having an opening 1a on a central part, the sub-reflection mirror 2 disposed to face oppositely on the optical axis of the main reflection mirror 1, a support 3 supporting and fixing between the main reflection mirror 1 and the sub-reflection mirror 2 at a prescribed interval, reference light sources 4, 5 attached to the peripheral part of the sub-reflection mirror 2 in axial symmetry, a half prism 6 arranged on the rear surface side of the main reflection mirror 1 coaxially with the opening 1a, the reference light source 8 arranged on the optical axis of the half prism 6, a quadrant photodetector 9 having plural photoelectric conversion sensors 9a-9d dividedly arranged coaxially with the opening 1a of the main reflection mirror 1, the half prism 6 and the reference light source 8 and an angle detection processing circuit 10 operation processing level differences of output voltages outputted from respective photoelectric conversion sensors 9a-9d of the photodetector 9.