A backlight unit has an external electrode fluorescent lamp in an housing. The external electrode fluorescent lamp includes a glass bulb having a discharge space inside, and electrodes around both ends of the glass bulb. A protective layer and a phosphor layer are formed on an internal surface of the glass bulb in this order. The glass bulb is made of soda glass, and sodium oxide precipitated from this soda glass appears on part of the internal surface of the glass bulb where the protective layer is not formed, so as to be exposed to the discharge space.