A lens for light emitting diode (LED) light sources which allows light emitted from an LED light source to exit the lens in a direction perpendicular to a vertical center axis of the lens. The lens of the present invention includes an inner space which is defined in a lens body having both a bottom surface and an upper reflective surface, so that light passing through the inner space is partially reflected by total internal reflection on a portion (selective transmission surface, inner reflective surface, inside reflective surface) of a boundary surface between the inner space and the lens body. Thus, light emitted from an LED light source efficiently exits the lens through a side surface. Accordingly, the lens of the present invention is used in efficient display and illumination optical systems.