A filament of simple structure showing improved conversion efficiency is provided. There is provided a light source device comprising a light-transmitting gas-tight container, a filament disposed in the light-transmitting gas-tight container, and a lead wire for supplying an electric current to the filament, wherein the filament consists of a single crystal. Sum of concentration of lattice defects and impurity concentration of the filament consisting of a single crystal is preferably lower than 0.01%.