A planar illumination device has a light source that shines light into a light guide plate through one side face (i) thereof and a reflector member laid on the back face of the light guide plate opposite to the light emission face thereof. In the vicinity of another side face (ii) of the light guide plate opposite to the side face (i) thereof through which the light is shone into the light guide plate, a slit is formed so as to extend along that side face (ii). A portion of the reflector member is, by being bent into an L shape, formed into an insertion portion, and this insertion portion is inserted into the slit.