A new system to precisely control the accelerating voltage of an electrostatic accelerator has been developed. With this development, the acceleration voltage stability ΔV/V in the low-frequency range was successfully controlled to 6.0 × 10−5. In this development, the cause of the voltage fluctuation of the acceleration voltage was investigated. As a result, it was found that noise components due to mechanical vibration existed. Therefore, the HVD (High-Voltage Divider) feedback method, which is less susceptible to mechanical vibration, was adopted and the effect of vibration was successfully eliminated. Furthermore, by adding a high-sensitivity CPO feedback system in place of the previously used slit feedback system, low-frequency voltage fluctuations were eliminated, and high acceleration voltage stability could be obtained even in beamlines without a slit system.