The object of the present invention is to achieve weight reduction of an illumination device and uniformization of luminance distribution. The illumination device (backlight unit) comprises a linear light source, a reflecting layer and an outlet layer. The reflecting layer and the outlet layer face each other via space. The linear light source is arranged at an end of the space. The outlet layer is formed as a semi-transmissive layer which transmits part of incident light while reflecting part of the incident light. The cross-sectional shape of the reflecting layer has an apex, which is concave in an illuminating direction, in a section from a position close to the linear light source to a central position of the reflecting layer and an inflection point in a section from the apex to a distal end of the reflecting layer.