A backlight device for a liquid crystal display comprising a light diffuser, a light source disposed behind the light diffuser, and a reflector for reflecting light from the light source, in which a liquid crystal panel is disposed in front of the light diffuser and irradiated, from the back side, with the direct light from the light source and the light reflected by the reflector and diffused by and passed through the light diffuser. The light source is of a type which radiates UV-rays and heat along with visible light. The warp and yellowing of the diffuser are suppressed even if the quantity of UV-rays is increased with an increase in the quantity of light from the light source. In a direct-under type backlight device for a liquid crystal display employing the light diffuser, the color difference (.DELTA.E) defined in JIS K 7105 after exposed for 500 hours to artificial light employed for artificial light source test defined in JIS h7350-2 is 2.0 or less, and saturated water absorption rate of the light diffuser is 0.9% or less.