A backlight device is provided that realizes high luminance while widening the color range. The backlight device illuminates a transmissive color liquid crystal display panel with white light from a back side of the transmissive color liquid crystal display panel having a color filter made of a three-primary-color filter which selectively transmits light having wavelengths of red light, green light, and blue light. The backlight device includes a plurality of cold cathode fluorescent lamp tubes having tube walls coated with a wide-color-range fluorescent material. The backlight device also includes a diffusion plate which diffuses white light emitted from the cold cathode fluorescent lamp tubes, illuminates the color liquid crystal display panel with the diffused white light, and is positioned at a predetermined distance from the cold cathode fluorescent lamp tubes. A predetermined pattern is printed on the diffusion plate, by dot printing, corresponding to positions at which the cold cathode fluorescent lamp tubes are provided, and the diffusion plate is positioned several mm apart from the cold cathode fluorescent lamp tubes.