In a surface-area light source device, a light entrance side surface of a light guide plate of a surface-area light source device has a staircase shape having steps in the vertical direction, and light emission diodes are provided at each step. The structure enables the distance between a light emission diode provided at one step and light emission diodes provided at an adjacent step to be maintained greater even if the thickness of the light guide plate is reduced so as to decrease intervals of the light emission diodes in the direction of the thickness of the light guide plate. As a consequence, the amount of transmission and reception of heat produced by the light emission diodes is reduced to prevent an increase in the temperature of the diodes. This allows a reduction in the thickness of the light guide plate with the brightness of light emitted from the light guide plate maintained, enabling the surface-area light source device to be thinned. As a result, a large number of light emitting diodes are provided, so that the brightness of the surface light source device is increased.