[en] An overview on four-year operation of the negative-ion-based NBI (N-NBI) system for the Large Helical Device (LHD) is presented, and prospects of negative-ion technology for applying it to future helical fusion reactors are discussed from theses experiences. The N-NBI system was designed and constructed based on the results of R and D at NIFS. Using three beam lines, the total port-through injection power of 9 MW for 2 s has been achieved. Each beam line attained the same performance: 3.5 MW of the port-through power with the beam energy of 165 keV for hydrogen, which corresponds to the averaged negative ion current density of 25mA/cm² from ion sources. Also for long pulse injections, 110 s for 0.1 MW by one ion source, and 80 s for 0.5 MW by two ion sources (one beam line) were carried out. 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 getting these successful results are described, and the problems that limit the present performance are clarified. Some R and D programs to solve these problems are proposed for further improvement. (author)