To provide a back light device suppressing the generation of luminance irregularity and having little luminance irregularity, by alternately arranging high voltage sides and low voltage sides of plural fluorescent tubes.

First to a fourth fluorescent tubes 2a-2d are mounted so that the high voltage sides and the low voltage sides are alternately arranged. When the first to the fourth fluorescent tubes 2a-2d are turned on, on the left side of a back light device 1, high luminance on the high voltage side of the first and the third fluorescent tubes 2a, 2c and low luminance on the low voltage side of the second and the fourth fluorescent tubes 2b, 2d are mutually canceled and, on the right side of the back light device 1, low luminance on the low voltage side of the first and the third fluorescent tubes 2a, 2c and high luminance on the high voltage side of the second and the fourth fluorescent tubes 2b, 2d are mutually canceled. Consequently, luminance on both left and right side parts are averaged and the back light device 1 having almost uniform brightness over a whole surface of a diffuser plate 4 is obtained.