A plane-shaped lighting device has a light-directing plate and a light source that is placed in contact with or in the proximity of the light-incident surface of the light-directing plate, and the light source is constituted of a plurality of LED lamps each of which has an LED chip for emitting light and a semispherical lens for directing the light from the LED chip into the light-directing plate. Here, the distance from the LED chip to the top of the semispherical lens is set to be 1.8 times as long as the radius of curvature of the semispherical lens. This arrangement allows the light emitted from the LED chip to be efficiently incident on the light-incident surface of the light-directing plate, thereby improving the light-incidence efficiency to the light-directing plate. Further, the application of this plane-shaped lighting device to display devices makes it possible to improve the efficiency of light utilization, and consequently to provide thinner display devices with high visibility.