An emission face of a light guide plate is divided into two portions, a first emission part having a large area and a second emission part having a small area, by a straight line extending from a generally lateral-center position of the incidence face perpendicularly to the incidence face. A slope is formed on the emission face in the vicinity of the incidence face. The slope is formed so that thickness of the light guide plate decreases from a side end portion of the first emission part toward a side end portion of the second emission part in a cross section vertical to the straight line. Further, thickness of the light guide plate may decrease gradually according to an increasing distance along the straight line from the incidence face.