Provided is an edge-light illuminating device in which a deformation is prevented from occurring in a reflection sheet due to thermal expansion of the reflection sheet. An edge-light illuminating device (3) of the present invention is arranged to illuminate a liquid crystal display panel (2) from a back side of the panel, and includes a light guide plate (7), a light source (5) that faces an edge face (72) of the plate and is arranged to project light onto the edge face, a case (4) arranged to house the plate and the light source, and a reflection sheet (6) laid on a bottom face (41) of the case and sandwiched between the bottom face and a back face (71) of the plate housed in the case, wherein the reflection sheet includes a slot portion (61) disposed along a direction that the reflection sheet expands by heat generated by the light source.