The invention provides a novel method for improving the surface properties or, in particular, the susceptibility to adhesive bonding of a shaped article of a synthetic polymer having aromatic rings and nitrogen atoms in the backbone chain of the polymer molecule, such as polyimides, polyamidoimides and polyaramides, so that the mechanical strengths of a resinous composite material reinforced with such a shaped article, e.g. fibers and cloths, may be greatly improved. The method comprises subjecting the surface of the shaped article to exposure to low temperature plasma of an inorganic, in particular, oxidizing gas generated by the glow discharge between electrodes under a reduced pressure, the discharge voltage between the electrodes being at least 4000 volts.