In this study, the authors present a new method for electrical potential observation in which helium ion microscope (HIM) observations can be performed while applying a selected voltage (up to approximately +5V) to a sample. The in situ voltage-application system is operated in a high vacuum HIM chamber and consists of a transfer rod, tilt joint, x-, y-, and z-axis mechanisms, and two probes at the end of the transfer rod. The new system was used to acquire a secondary electron (SE) images of the cross-sectional surfaces of a multilayer ceramic capacitor (MLCC) with voltages (from 0.5 to 5 V) applied to the internal electrodes of the MLCC. The active voltage contrast corresponding to the electrical potential in the SE images could be observed when the applied voltage was less than or equal to 1.5 V. This technique enables any voltage to be applied to the samples from outside the chamber and can be used to measure the nanometer-scale electrical potential distribution of Li-ion rechargeable batteries, solar cells, etc. Published by the AVS.