We propose a polarization-maintaining waveguide fabricated with a large waveguiding area (core diameter: 750 mm) using polymeric material. We previously developed and demonstrated a zero-birefringence copolymer in a film structure of 40 to 70 mm thickness. This time, we have fabricated a 20-m-long optical fiber using the same copolymer as the core material. The polarization-maintaining performance of the low-birefringence polymer optical fiber was studied by comparison with that for other types of plastic optical fibers. As a result, the low-birefringence polymer optical fiber showed a high extinction ratio for fiber lengths between 0.4 and 20 m.