To improve luminance in a lighting system having a light guide plate and a reflection sheet.

A backlight device includes a light guide plate 20 having an incident surface fitted on a side surface, an emitting surface fitted on one of plate surfaces, and an opposite plate surface 20c being a plate surface on the opposite side to the emitting surface; an LED light source arranged opposite to the incident surface of the light guide plate 20; and a reflection sheet 26 arranged on the opposite plate surface 20c side of the light guide plate 20, and forming a reflecting surface for reflecting light with a surface opposed to the opposite plate surface 20c, the reflection sheet 26 with the reflecting surface forming an irregular shape including a slope 26c1 sloped and rising from the LED light source side to the opposite plate surface 20c side.