The invention relates to an illumination device comprising a light-emitting tube having a pair of electrodes and a reflecting mirror for reflecting the light from said light-emitting tube toward an illumination region. Said reflecting mirror has an inner surface and an outer surface and said inner surface is inclined at an angle θ with respect to an ideal inner surface, said ideal inner surface reflects the light from an arc image generated between the pair of electrodes so that the reflected light passes through the arc image. Furthermore, a reflecting film of the reflecting mirror is provided on an outer surface of said reflecting mirror.