According to one embodiment, a backlight device includes a light guide including an emission surface including a flat area and an inclined area inclined in an out-of-plane direction from an edge of the flat area, a light source emitting light into the light guide, and an optical diffusion sheet on the emission surface. The optical diffusion sheet includes a diffusion flat area overlapping the flat area of the emission surface and a diffusion inclined area overlapping the inclined area of the emission surface, and a haze of the diffusion inclined area is higher than that of the diffusion flat area.