Gas discharge is caused in a predetermined discharging gas at atmospheric pressure or a pressure close to atmospheric pressure, and an organic material which is liquid at room temperature and which is previously contained in the discharging gas or applied to a surface of a treated member is dissociated or excited by a plasma caused by the gas discharge to generate activated species. By using these excited activated species, a polymerized film of organic material is formed on the surface of the treated member. By variously selecting the organic material or the kind of the discharging material and variously combining them, a water repellent film, a hydrophilic film or a film having a high hardness can easily be formed on the surface of the treated member according to use, or the polymerization speed of the organic material can be increased, or the polymerization can be limited. Further, the adhesion of an organic polymerized film essentially low can be improved with respect to a treated member formed of an inorganic material such as glass or some inactive organic material if the organic material or the kind of gas is used while being changed.