An optical apparatus for detecting the position of an object by projecting a light image onto the object through a transparent plate and detecting the reflected light from the object through the transparent plate is disclosed. The projection optical system includes a first objective lens for forming the light image. The optical axis of the projection is disposed inclined to the above-said transparent plate. The detection optical system includes a second objective lens for refocusing the light reflected from the object. The optical axis of the reflection light detection between the second object lens and the object is disposed inclined to the transparent plate. The apparatus further comprises plane parallel optical members mounted obliquely so as to cancel the asymmetric aberrations generated by the above-said transparent plate. One of the aberration-correcting plane parallel optical members is interposed in the projection optical path on the side opposite to the transparent plate relative to the first objective lens. The other one is interposed in the detection optical path on the side opposite to the transparent plate relative to the second objective lens.