Provided is a back light unit of an edge light type, including at least a light guiding plate, a lamp, a reflection sheet and a reflector. The lamp is disposed on the side surface side of the light guiding plate. The reflection sheet is disposed on the back surface side of the light guiding plate. The reflector supports the reflection sheet from the back surface of the reflection sheet, and reflects light from the lamp towards the light guiding plate. The back light unit has a structure in which the reflection sheet is caused to come into contact with a ridge where the side surface of the light guiding plate and the back surface thereof meet, by curving an end of the reflection sheet on the side of the light source towards the side of the light guiding plate.