In a method of forming a thin light guide plate which includes a compression step in an injection molding step, an optical pattern is liable to be adhered to a mold. Further, in removing the light guide plate from the mold using an ejector pin, a stress is concentrated on a local area of the light guide plate thus generating warping, deformation or irregularities in size of the light guide plate. To overcome such drawbacks, a liquid crystal display device is configured such that an optical pattern portion is compressed, and the light guide plate is removed by making use of a peripheral portion of a mold thus preventing the generation of stress in a local area of the light guide plate due to an ejector pin.