The invention provides an illumination unit for liquid crystal display systems, which comprises an illumination light source for emitting visible light rays and a light guide plate or a light diffusion plate having an entrance surface and an exit surface. The illumination light source is located such that light rays therefrom are entered into the light guide plate or light diffusion plate from the entrance surface. The light guide plate or light diffusion plate is formed of a transparent synthetic resin. The maximum intensity of light in an ultraviolet range having a wavelength of 350 nm or less as measured all over the entrance surface of the light guide plate or light diffusion plate is 2 W/(cm²nm) or less. The illumination light source has a maximum emission intensity of 2 W/(cm²nm) or less in an ultraviolet range having a wavelength of 350 nm or less.