The lateral resolution of the angle-based deflectometric surface profiler using a commercially available autocollimator has been improved by introducing a novel null instrument. The proposed null instrument is simple, inexpensive, and has a short response time. High-accuracy flatness measurements of low-reflective surfaces have been successfully demonstrated using a laser beam with a spot size of 1 mm. The repeatability of the surface profile measurement is better than +/- 0.6 nm.