<p>Optical tweezers have played an important role in various scientific fields, from biology to nanotechnology. Back-focalplane interferometry is one of the widely used detection methods for optical tweezer and allows us to detect a trapped particle with nanometer resolution. We proposed a novel improved back-focal-plane interferometry acquiring only surrounding area of detection beam. To verify the validity of the proposed method we develop fundamental apparatus based relay lens system. A verification experiment employing 90-nm TiO<sub>2</sub> particle confirmed that the proposed method could improve a detection sensitivity about 2 times higher.</p>