a mirror rotation angle detection mechanism is provided for an optical system having a mirror which is rotatable about an axis substantially on a reflection surface thereof at which a main beam of light is reflected . the mirror has a thin reflective layer with opposite surfaces that are nearly coplanar , and one of which constitutes the main beam reflection surface . the detection mechanism includes a light emitting member to irradiate a beam of detection light onto the opposite surface of the reflective layer . a light receiving element recieves the reflected detection beam , and an angle detection circuit detects the rotation angle of the mirror in accordance with the output of the light receiving element . according to a preferred embodiment , a condenser lens may be arranged between the light emitting member and the detection beam reflection surface .