A negative-ion-based neutral beam injector is designed conceptually for Fusion Engineering Reactor (FER). The injector is characterized by a long, slender beamline, which is 0.3-0.4m wide, 40m long, supplying 500keV, 20MW neutral beams into the tokamak. Beam divergence is 0.3 degree and such a low divergence is considered to be attainable by using a volume D-ion source because the divergence of 0.8-0.9 degree was obtained in our recent experimental results in spite of the low beam energy of 25keV. Tangential beam injection is possible in this design because of high beam energy and a narrow drift duct. Maintenance of the injector will be easy since beamline components installed near the tokamak are small and simple, and complicated beamline components such as the ion source are situated far from the tokamak.