A backlight unit includes: a wiring board having a light source mounted thereon; a light guide plate; a light-shielding pressure-sensitive adhesive sheet; and a light diffusion sheet. The front surface of the light guide plate includes an effective region serving as the planar light source and a light entering region ranging from the end surface into which the light enters to the effective region. The wiring board and the light-shielding pressure-sensitive adhesive sheet each have a part positioned in the light entering region. The light diffusion sheet is arranged from the effective region to the light entering region, and overlaps with the light-shielding pressure-sensitive adhesive sheet in the light entering region. On a side closer to the effective region with respect to the light-shielding pressure-sensitive adhesive sheet, the light entering region is covered with the light diffusion sheet without a gap.