The operation of an accelerator at low pressure is an essential requirement to reduce the stripping loss of negative ions, which, in turn, results in high efficiency of the neutral beam systems. For this purpose, a vacuum insulated beam source (VIBS) has been developed at Japan Atomic Energy Research Institute, which reduces the gas pressure in the accelerator by enhanced gas conductance through the accelerator. The VIBS achieves a high voltage insulation of 1 MV by immersing the whole structure of the accelerator in vacuum with a long (∼1.8 m) insulation distance. Results of the voltage holding test using a long vacuum gap of 1.8 m indicate that a transition from vacuum discharge to gas discharge occurs at around 0.2 Pa m in the long vacuum gap. So far, the VIBS succeeded in accelerating a 20 mA (H−) beam up to 970 keV for 1 s. It has been demonstrated that the high voltage holding capability of the 1 MV bushing surrounding the VIBS accelerator could be drastically improved by installing new large stress rings that reduces the electric field concentration at the triple junction. After implementing this change, the VIBS sustained 1 MV stably for more than 1200 s. Acceleration of ampere class H− beams at high current density is to be started soon to demonstrate ITER relevant beam optics.The operation of a negative ion source at low pressure is also essential to reduce the stripping loss. However, it was not very easy to attain high current density H− ions at low pressure, since the destruction cross-section of the negative ion becomes large if the electron temperature is >1 eV in low pressure discharge. Using a strong magnetic filter to lower the electron temperature, and introducing higher arc discharge power to compensate for the reduction of plasma density through the filter, an H− ion beam of 310 A m−2 was extracted at a very low pressure of 0.1 Pa. This satisfies the ITER requirement of current density at one-third of the ITER design pressure (0.3 Pa).