PURPOSE: To uniformalize the film thickness between the upper and the lower parts of a glass tube, by drying almost the upper part of the glass coated with a phosphor, then letting flow away the amount of aqueous paint in the lower part of the glass tube through stopping the air blow for drying for a while, and afterward again starting the blower drying.

CONSTITUTION: A phosphor aqueous paint is applied from the upper part onto the internal surface of a glass tube which is vertically held, thereafter a hot blast is sent from a blow nozzle 1 in order to dry a phosphor film while the glass tube is held by a tube chuck 2. For 1W3min of the beginning, the hot blast is sent at the flow rate of 50W120l/min and at the temperature of 60W90°C to dry the glass tube. Next, the air blow is completely stopped and the excess amount of the aqueous paint between the middle part and the lower part of the glass tube is forced to flow down to adjust the distribution of the film thickness. If the period of stop thereof is long, the film thickness of the middle part becomes too small; so said period of stop is desired to be practically 1W3min, preferably 1W2min. After that, the drying is continued by sending the same hot blast which is used for drying the upper part. In this case, forced drying caused by thermal radiation using a gas burner or the like from the external surface (side face) of the glass tube may be also used together with the above drying in order to shorten the total drying time required.