To reduce moisture in pores of a carbonaceous material and surface functional group by subjecting a carbonaceous material, having a specified specific area to surface treatment with halogen substituted aliphatic hydrocarbons or halogen substituted aliphatic ether, before being impregnated with organic electrolyte.

A carbonaceous material, having specific area of 100-3,000 m2/g, e.g. high purity phenol resin based active carbon powder, a conductive material, and a binder are mixed and kneaded, while dripping ethanol and then it is rolled and dried to produce an electrode sheet. The electrode sheets are bonded through conductive adhesive to the opposite sides of an aluminum foil current collector which has a lead-out part and then is roll pressed to produce positive and negative electrode bodies. The positive and negative electrode bodies are inserted into an aluminum case and a processing agent comprising fluorine substituted aliphatic hydrocarbon is injected into the case. The case is then placed in an enclosed container, pressure in the container is reduced, and after the active carbon is impregnated with the processing agent in a liquid state, heat treatment is conducted in the atmosphere.