PURPOSE: To obtain a resin-coated substrate having a uniform surface condition and a uniform film thickness, by coating the substrate with a polyamic acid varnish, removing a solvent contained in the film by heating the film at specified temperatures, smoothing by heat-pressing the film formed with a heating roller, and thermal-treating the film to convert the polyamic acid into polyimide.

CONSTITUTION: A substrate 2 is coated with a polyamic acid varnish, and a solvent contained in the polyamic acid varnish film 1 formed on the substrate 2 is removed by heating the film at temperatures (e.g., 60-120°C) where the polyamic acid can not be converted into polyimide. The residual polyamic acid varnish film 1 is smoothed by heat-pressing the film directly or through a releasing sheet 7 with a heating roller 4. The smoothed polyamic acid varnish film 1 is then converted into a polyimide film by the thermal treatment at temperatures between 120°C and 480°C to form a polyimide covering layer. In this way, a resin-coated substrate having a uniform surface condition and a uniform thickness is obtained.