According to one embodiment, an illumination device includes a light guide including a first main surface, a second main surface, a first side surface, and a second side surface, a plurality of convex portions provided on the second main surface, and a first light source element and a second light source element opposing the first side surface and the second side surface, respectively, and when an incident angle of a main ray of the light propagating in the light guide is expressed by θ, a diffusion angle of the light is δ, an angle of the convex portion of the light guide is α, a critical angle of the light guide is θc, and an angle which satisfies (90°−θc) is β0, a relationship: ((β0+δ)/3)