To provide a discharge lamp lighting device and a backlight unit, capable of detecting abnormalities of a double tube discharge lamp.

In this backlight unit 1, a double-tube discharge lamp 2, a temperature sensor 3, a luminance sensor 4, a liquid crystal display part 5, a light- transmitting plate 6, an unillustrated control means and a lighting device are constituted in a unit shape. This embodiment fetches the output of the temperature sensor 3 and the luminance sensor 4, detects whether the output has become a prescribed luminance value set by an arithmetic circuit, detects it as an abnormal state of impairing airtightness of a arc tube or an outer tube of the double- tube discharge lamp 2, when this detecting result causes errors from a preset luminance value, and detects it as an abnormal state of impairing airtightness of the arc tube or the outer tube of the double-tube discharge lamp 2, when a detecting result of lamp voltage causes errors from a preset lamp voltage value.