A slow positron beamline using a nuclear reactor is under development at the Kyoto University Research Reactor (KUR). A brightness enhancement system was designed to reduce the initial positron beam size (similar to 30 mm) below typical specimen sizes (< 10 mm) and keep the intensity of the beam as high as possible. After installation of the brightness enhancement system at the beamline, we tested its optical performance using an electron beam. The experimental result indicated that the system would have enough capability to obtain a small-sized beam suitable for positron measurements.