PURPOSE: To obtain the excellent adherence to resin-containing sheet, unification and heat resistance by a method wherein polymerization initiator and polyfunctional monomer having polymerizable unsaturated group are held in the form bondable to rubber sheet.

CONSTITUTION: Polymerization initiator and polyfunctional monomer having polymerizable unsaturated group are held in the form bondable to rubber sheet, to which thermosetting resin containing sheet used for capacitor packing and the like is laminated. Under the condition that electrolyte is volatile polar solvent having strong dissolving power such as N, N-dimethyl formaldehyde or the like, as the rubber for the rubber sheet, partially crosslinked butyl rubber is specially preferable. Further, the vulcanized rubber is preferable for the rubber sheet. The preferable polymerization initiator is organic peroxide such as lauroyl peroxide or the like. As the polymerizable unsaturated group of the polyfunctional monomer, aryl group or the like is preferable. As the bondable form, the forms of monomer, the lower polymer of the like of monomer are exampled.