A planar illumination device includes: a light guide plate formed by arranging a plurality of unit light guide plates each having a light emission surface as a front surface and a reflection surface as a back surface so that facing end surfaces of adjacent unit light guide plates of the plurality of unit light guide plates are closely arranged; and one or two or more light sources arranged on a side surface, in a direction where the plurality of unit light guide plates are arranged, of each of the plurality of unit light guide plates, in which a projection section is arranged on one of facing surfaces of two adjacent unit light guide plates in the arrangement direction and a recession section in which the projection section is engaged is arranged on the other of the facing surfaces of the two adjacent unit light guide plates.