The five-year operation of the negative-ion-based neutral beam injector (N-NBI) system on the large helical device (LHD) is reviewed, and the prospects of negative-ion technology for applying it to future helical fusion reactors are discussed from these experiences. The N-NBI system was designed and constructed based on the results of R&D at NIFS. Using three beamlines, the total port-through injection power of 10.3 MW for 2 s has been achieved. Each beamline attained almost the same performance: similar to3.5 MW of the port-through power with the beam energy of similar to165 keV for hydrogen, which corresponds to the averaged negative ion current density of 25 mA cm(-2) from each ion source. Recent modification of accelerators in one of the beamlines was very successful to increase beam energy up to 180 keV and port-through power, 4.4 MW. Long pulse injection heating was also pursued under the steady state LHD operation, and 110 s for 0.1 MW by one ion source, and 80 s for 0.5 MW by two ion sources (one beamline) were achieved. These facts show that the negative ion technology has been established to the same power level of conventional positive ion systems. Several technical improvements needed for obtaining these successful results are described, and the problems that limit the present performance are clarified. Some results of recent R&D to solve these problems are shown.