PURPOSE: To prevent disturbance of a boundary of layers in film forming by providing hot air drying coating with application material as water-soluble binder except to at least an outsidemost layer, which is reformed into water-soluble material by heat.

CONSTITUTION: An entire bulb is submerged into the first application liquid, picked up and then given hot air drying by blowing 90-110°C hot air. As water is less vaporized, the temperature of an application film rises so that deammoniation condensation occurs in acrylic copolymer as binder to form the first water-nonsoluble application film on a continuous basis. Next, the side semi-peripheral portion is submerged into the second application liquid and picked up for single-face application. Since the first application film binder has somewhat hydrophilic property, uniform coating is provided with the second application liquid without being repelled and still no melting-out of film component occurs on the surface of the first application film and no mixture of titanium oxide powder into the second application liquid occurs. Drying is effected by blowing 90-110°C hot air.