Hydrogen negative ion beams of 507 keV, 1 A and 486 keV, 2.8 A have been successfully produced in the JT-60U negative ion source with a three-stage accelerator by overcoming a poor voltage holding of the accelerator with large-size grids of ∼2 m2. This is the first result of H− beam acceleration up to 500 keV at a high current of over 1 A. In order to improve the voltage holding capability, the breakdown voltages of the large-size grids and small-size electrodes with uniform and locally strong electric fields were examined by changing the gap length. It was found that the voltage holding of the large-size grids was below half of that of the small-size electrodes with a uniform electric field which was used in the design of the accelerator. This degradation was found to be caused by the local electric field concentrations in addition to the size. Based on the results of the voltage holding tests and beam optics calculations, the gap lengths of the large-size grids were tuned to have a capability to sustain 600 kV. As a result, the gap tuning realized stable voltage holding during beam accelerations without significant degradations of the beam optics and stripping loss. These results indicated that stable 500 keV beam accelerations required for JT-60SA are feasible and this gap tuning is also applicable for the design of ITER accelerator.