PURPOSE: To treat the surface of a substrate uniformly and beautifully in a gas phase with an ultra-thin film excellent in hydrophilicity, antistatic property and mechanical strength, by plasma-polymeriziang a caboxylic acid such as (meth) acrylic acid on the surface of the substrate and after-treating it with NH3 gas.

CONSTITUTION: A substrate 2 such as woven polyester fabric is placed within a reaction tube 1 and the pressure in the tube 1 is decreased to 10-3W1.0 Torr by means of a rotary pump 7 and a diffusion pump 6. An equimolar gas mixture comprising vapor of (meth) acrylic acid monomer sent from a monomer reservoir 10 and regulated by a needle valve 8 and CO2gas sent from a CO2 gas cylinder 13 and regulated by a needle valve 8' is fed through an inlet 12 to the tube 1 and plasma-polymerized at a temperature of 20°C for about 1 min by starting discharge by using an induction coil and a high-frequency electric source and thereby generating energy of 50W400 MJ per kg of the carboxylic acid. After completion of the plasma polymerization, the tube 1 is evacuated and fed through the inlet 12 with NH3 gas sent from a NH3 gas cylinder 14 through valves 9' and 8', and the substrate is after-treated with this gas for at least 30 min at a pressure ≥10 Torr.