PURPOSE: To remove foreign substances which adhere to the inner surface of a bulb by forming phosphor, lacquer, metal-back, and internal conductive film in the bulb and thus forming a protective film which can be removed by means of thermal decomposition at a neck section, and burning them later.

CONSTITUTION: Phosphor, lacquer, metal-back, and internal conductive films 5, 6, 8, and 7 are formed in a bulb 4. Before burning these films, a protective film which can be thermally decomposed and scattered by the burning temperature, for example, acrylic resin lacquer film is formed on the internal conductive film 7 which is adjacent to the internal surface of a neck section 1 and that of the neck section 1 of a funnel section 2. Then the bulb 4 is but in a calcimation furnace and burnt. The lacquer film is thermally scattered and a protective film 12 is thermally decomposed. As a result, even when foreign substances, such as phosphor and graphite adhere to the internal surface of the neck section, they can be separated and easily removed simultaneously with the decomposition of the protective film.