A backlight (2) of the present invention is a tandem backlight. This backlight (2) includes: a light source (5); a plurality of light emitting units (11) each including a light guide body (7) that diffuses light from the light source and causes the diffused light to exit from a light emitting surface; and a substrate (4) on which the light source (5) is fixedly connected and which is connected to the light guide body (7). On a surface of the light guide body (7) which surface faces the substrate (4), a protrusion (12) for alignment with the substrate is formed. Meanwhile, in the substrate (4), a through hole (13) into which the protrusion (12) is engaged is formed. An engaging section B (alignment section) of the protrusion (12) and the through hole (13) is disposed so as to be closer to a light emitting section (7b) than the connecting area A of the light guide body (7) and the substrate (4). According to the present invention, in a tandem illumination device, highly precise alignment of the light source and the light guide body can be performed. As a result, luminance uniformity can be improved. In addition, a defect in mounting of the light guide body can be easily distinguished.