To provide a thin backlight device for a liquid crystal display aiming at a large liquid crystal display which is large in size, high in brightness, and uniform in surface brightness.

Lamp housing grooves 11 having a curved roof face 11a and a linear wall face 11b are formed at not less than 4 places of the rear face of an acrylic light guide plate 1, a light reflecting face 12 with a step-wise slant is formed at the both sides of respective lamp housing grooves 11 except a flat plane part 1b of a small width. A plurality of V grooves 1a are formed at the whole front face of the guiding light plate 1 with a prescribed V groove forming angle xy, and a cold-cathode fluorescent lamp 2 is mounted in the lamp housing grooves 11. By having this shape, a lighting light of the cold-cathode fluorescent lamp 2 is divided into incident light to the curved roof face 11a and incident light to the linear wall face 11b, and it irradiates the inner face of the V groove 1a of the same range with an angle of not more than 42 degrees to its normal, and passes through the V groove 1a without being reflected on the inner face of the V groove 1a, and thereby the problem is solved.