To provide a backlight that reduces the number of LEDs used while facilitating the attempt to make the backlight smaller in thickness. On amount substrate (11), LEDs (13) are mounted in a square lattice arrangement. Over a portion near the center of each unit of the square lattice, protrusions (15) of a diffusion plate (14) are disposed. Among light emitted from the LEDs (13), light traveling in lateral directions between the mount substrate (11) and the diffusion plate (14) is captured by the protrusions (15). The captured light is refracted and reflected by the interfaces of the protrusions (15), and diffused due to diffusing particles, with the result that the light is turned into upward illumination light.