A method for machining by Electrical Discharge Machining (EDM) on an insulating material is proposed in this paper. The authors modified a equipment for holding electrode by fitting a rotating disk as the electrode(RDE). EDM have been succeeded to with the assisting electrode method(AEM). The process has been demonstrated by producing a slot on a silicon nitride. Main objective is feasibility study to make slot width less than 1 mm and depth 1 mm. The electrode tool was made from a thin brass disk with a thickness of 0.75 mm. The results can be evaluated from product appearance, Material Removal Rate(MRR), Electrode Wear Ratio(EWR) and Surface Roughness. The conductive layer was examined with a Scanning Electron Microscope (SEM) with Energy Dispersive Spectrometers (EDS).