Abstract: A light source device has a transparent plate having a smoothened front surface and a roughened rear surface. A light source is disposed adjacent to an end face of the transparent plate. The roughened rear surface has a portion which is inclined and curved and disposed adjacent to the end of the transparent plate remote from the light source. A reflecting plate is disposed adjacent to and extends along the roughened rear surface of the transparent plate. The provision of the inclined and curved surface portion assures that the luminance of the light on the smoothened front surface of the transparent plate is substantially uniform over the entire surface area. The inclined and curved surface portion is represented by the following equation,y=H--(H-h)x.sup.n /L.sup.