The side surface of the light guide plate includes an incident surface which faces the light source unit in an opposed manner, and a fixing surface which is directed in the left-and-right direction orthogonal to the top-and-bottom direction which is the direction along which the light source unit and the incident surface each other in an opposed manner. An engaging portion which is engageable with a lower frame is formed on the fixing surface or an edge portion which forms the fixing surface. The engaging portion is configured to allow a change of a distance between the incident surface and the lower frame due to expansion or shrinkage of the light guide plate while restricting the movement of the light guide plate in the top-and-bottom direction.