This paper reports the progress of R&D towards the ITER neutral beam (NB) system at the Japan Atomic Energy Agency (JAEA) during 2004–2006. In the accelerator development, an H− ion beam of 146 A m−2 was accelerated up to 836 keV in a vacuum insulated accelerator with five acceleration stages. This is the first production of a high power density beam at MeV range energy, which is more than twice as high as those of the existing NB systems for large experimental reactors such as JT-60U and LHD. The ITER NB system requires a high voltage (HV) bushing as an electric feedthrough to the vacuum insulated accelerator from the gas insulated power supply. A full-size (1.56 m in diameter and 0.292 m in height) insulator ring made of alumina ceramic has been successfully fabricated for HV bushing, as a result of the technological development of a new cold iso-static press.Long pulse operations of JT-60U negative ion sources with a large extraction area of 0.45 × 1.1 m2 are in progress. From the two ion sources, D− ion beams of 21 A (90 A m−2), 320 keV have been produced for 21 s. As a consequence, injection of 3.2 MW D0 beams was achieved. Thus, the long pulse operation of the negative ion based NB system has been demonstrated with multi-tens of ampere of D− ions extracted from the ion extraction area relevant to the ITER NB design (0.6 × 1.5 m2). For long pulse operation of such powerful beams, the negative ion uniformity over the wide extraction area is an essential issue to avoid excess power loading on the accelerator grids. By adopting a new magnetic configuration with the so-called 'tent-shaped filter', the uniformity of the JAEA 10 A source (extraction area: 0.13 × 0.22 m2) was improved by 8%, which fulfilled the ITER requirement.