Provided is an optical apparatus using reflection geometry. The optical apparatus includes a lens element disposed to face an object to be measured, a light source generating an incident beam that passes through the lens element to be incident on the object, and a photodetector receiving light that is scattered by the object. The incident beam is obliquely incident on the object off an optical center axis of the lens element, without passing through the optical center axis. The scattered light is transmitted to the photodetector by passing through the optical center axis of the focusing lens element and a region therearound.