Abstract: Electrical elements constituting an electrical device are embedded into at least one layer of a finely divided insulating material which is formed within a case by filling the case with the insulating material. A low viscosity resin is poured into the case to be impregnated into said at least one layer of the insulating material and to form a cover layer covering the free surface of the top insulating cover layer. The resistances to moisture, vapor, shocks, vibration, cycling temperature change and so on are remarkably improved so that the highly reliable performances of the enclosed electrical device under severe environmental conditions may be ensured.