PURPOSE: To reduce the generation of uneven intensity of illumination on an illuminated face, by making temperatures of a damper and a thermion generating source almost equal at a contacting part in a light source device lighting a fluorescent body.

CONSTITUTION: The dampler 11 is constituted with an electric resistance material, through which a current flows so as to heat the damper itself. A voltage applied to the damper 11 is set so that the temperature of the damper 11 and that of the filament 9 are almost equal at the contact part K between them. The rising of the temperature of the damper 11 in this way, the trouble of the reducing the amount of emission of thermions at the filament part K is eliminated and the partial depression of the intensity of illumnation of the fluorescent body is eliminated or reduced.