R&D pertaining to a 1 MeV accelerator and a large negative ion source has been carried out at the Japan Atomic Energy Research Institute for the ITER neutral beam system. The R&D is, at present, progressing towards: (1) 1 MeV acceleration of H− ion beams at the ITER relevant current density of 200 A m−2 and (2) improvement of uniform negative ion production over wide extraction area in large negative ion sources. Recently, H− ion beams of 1 MeV and 140 mA level have been generated with a substantial beam current density (100 A m−2). In the uniformity study, it was clarified that the electron temperature in the ion extraction region was locally high (>1 eV), which resulted in the destruction of negative ions at a high reaction rate. Interception of fast electrons leaking through a transverse magnetic field called 'magnetic filter' has been found to be effective in lowering the local electron temperature, followed by an improvement of negative ion beam profile.