A planar illumination apparatus includes: a light source; a light guide plate having a light-receiving end surface arranged facing the light source, an output flat surface for planarly outputting light received through the light-receiving end surface and a reflective flat surface facing the output flat surface; a reflective sheet disposed on the reflective flat surface side of the light guide plate; and a flexible printed circuit board disposed on substantially the same plane as that of the reflective sheet, the flexible printed board mounting the light source, wherein the flexible printed circuit board has a mounting portion on which the light source is mounted and a thin portion which extends from a forward side edge and is formed thinner than the mounting portion, and wherein at least one part of the thin portion and an end portion of the reflective sheet are overlapped with each other.