An incidence face is formed at a corner portion of a light guide plate having a generally rectangular emission face, being supplied with primary light form a primary light source. The incidence face has a configuration which obtained by applying undulation-modification to an imaginary incidence face that cuts off the corner portion obliquely. This undulation-modification gives the incidence face first and second incidence regions. The first incidence region is provided with unevenness that produces an inner propagation light having a diverging angle greater than that which would be obtained under an imaginary case where primary light was refracted on incident to the imaginary incidence face. The second incidence region is provided with slopes inclined with respect to the imaginary incidence face, causing the emission face to have a reduced unbalance in brightness. The imaginary incidence face may extend in a direction perpendicular to an imaginary angle-bisector passing a corner point of an effective emission area. Both side faces adjacent to the incidence face may make an angle smaller than 90.degree..