PURPOSE: To reduce the temperature of heating process and obtain sufficient bonding strength by coating a fluorescent material suspension which is an aqueous solution of water soluble organic adhesive including sodium phosphate or borate on the inside surface of a glass tube.

CONSTITUTION: Sodium phosphate or borate is added to a fluorescent material suspension so that the quantity of sodium to the fluorescent material ranges from 15ppmW200ppm by weight, this suspension is coated on the inside surface of a glass tube, and the tube is heated at the temperature lower than 600°C to remove the adhesive. A fluorescent layer with sufficient bonding strength can be formed by heat treatment at the temperature of the adhesive removing process, thermal energy consumption is reduced, and the lowering of flux maintenance can be prevented.