An illumination device includes a plurality of light source units (20) each having a light guide plate and at least one light source. The light guide plate is provided with an illumination region (4) through which an incident beam of light from the at least one light source is emitted outward and a light guide region (3) through which the incident beam of light from the at least one light source is guided toward the illumination region (4), with the illumination region (4) and the light guide region (3) laid side-by-side. Light source units (20) adjacent to each other along a direction of an optical axis of the at least one light source are disposed so that the illumination region (4) of one of the light source units (20) covers at least a part of the light guide region (3) of the other light source unit (20). A diffuse reflection sheet (33) and a specular reflection sheet (32) or a light absorption sheet are provided between the light guide plate (LG(k)) of the one of the light source units (20) and the at least one light source (BL(k+1)) of the other light source unit (20) in such a way as to be put on top of each other. This makes it possible to suppress uneven irradiation and improve luminance uniformity and color-mixing properties of a tandem illumination device having light guide plates disposed in such a way as to be overlapped.