Epoxy / silica nanocomposite materials are attracting attention as a promising insulating material for next-generation electric rotating machines. It is empirically known the dielectric breakdown strength of an organic insulating material can be improved by mixing with nano-scale inorganic fillers. However, the role of the inorganic filler in the suppression of the dielectric breakdown is not fully understood yet. In this study, we are developing an efficient method for characterizing dielectric breakdown strength of epoxy / silica nanocomposite material, where the epoxy resin sample is thinned to sub-micrometer scale to induce the breakdown even at a low voltage and in a short time. The problem with using thin epoxy resin is its fragility; the thin epoxy resin is easily damaged by sticking an electrical probe on it. This problem is resolved by depositing a buffer metal layer on the epoxy resin and employing a metal bead as the electrical probe. It is confirmed by SEM observation that the damage at evaluated point on the epoxy resin is successfully suppressed. Using the bead probe, we successfully monitored the current transition of the dielectric breakdown events.