[en] Breakdown locations of a JT-60U negative ion source were investigated to improve the voltage holding capability. The accelerator is characterized by three acceleration stages with large grids 0.45 m x 1.1 m and large FRP insulators 1.8 m in inner diameter. High voltages were applied to each acceleration stage independently, and then breakdown locations were examined by CCD cameras. Voltage holding capabilities of each stage were almost the same, ∼120-130 kV, which was lower than the design acceleration voltage of 167 kV. Then, in order to identify whether the breakdowns occur in the gaps between grids or on the surface of the FRP insulators, high voltages were also applied to the accelerator with the grids and their support flanges removed. The voltage holding capabilities of three FRP insulators rapidly achieved 167 kV. These results indicate that the breakdowns mainly occur in the gaps between the acceleration grids and/or their support flanges. The detailed breakdown locations were identified from the light emission due to breakdown. In the first and second acceleration gaps, almost no light emission was observed when the breakdown occurred. This suggests that the breakdown might occur in the gaps between the grid support flanges that are situated out of the sight of the CCD cameras. In the third gap, the location of the light indicated that most of the breakdowns occurred in the gap between the grids. (author)