Discharge lamp includes a synthetic quartz glass tube having an inside diameter of 8 mm or over and a pair of filaments provided within and at opposite ends of the glass tube with an L (cm) filament-to-filament distance, and rare gas and metal including at least mercury are sealed in the interior of the glass tube. Lamp voltage V (V) and lamp current I (A) during illumination of the discharge lamp, filament-to-filament distance L (cm) and inside diameter D (cm) of the glass tube have relationship represented by the following mathematical expression. Namely, (V - Vf) / L = X / (√D * √I) and 2.6 ≦ X ≦ 4.2, where Vf is a constant factor depending solely on a illuminating power source and wherethat if the discharge lamp is illuminated by a high-frequency power source of 1 kHz or over, Vf is 10, but if the discharge lamp is illuminated by a power source of 1 kHz or below, Vf is 50.
