PURPOSE: To obtain a good insulating material for direct current gas insulation apparatus by forming, on an epoxy-cast insulating material surface, a thin film for reducing the surface resistivity to achieve only its reduction without degradation of the characteristics of the original material.

CONSTITUTION: An epoxy resin insulating material 3 is set on a table 2 in a plasma polymerization vessel 1, which is then evacuated through the air outlet 1a, followed by generation of a glow discharge by an inductive high-frequency discharge coil 5 while injecting an inert gas 4 through the inlet 16. A polar group-contg. monomer (e.g., methyl methacrylate) vapor 6 is fed, through the inlet 1C, into the vessel 1 so as to be hit by the glow discharge to form, on the surface of said material 3, a polymeric thin film of lower surface resistivity than that of the material 3.