A planar light-emitting device comprises a plurality of light-emitting units each including a lightguide plate and arranged adjacent to each other. Each lightguide plate has an upper surface as a light-exiting surface, a lower surface opposite to the upper surface, and a peripheral side surface extending between the peripheral edges of the upper surface and the lower surface, wherein a light-entrance surface is defined by a part of the peripheral side surface. The light-entrance surface has a first protruding portion extending from and along the light-entrance surface and an opposite surface opposing the light-entrance surface has a second protruding portions extending from and along the opposite surface. In each adjacently disposed pair of light-emitting units, the first protruding portion of the first light-emitting unit abuts against the second protruding portion of the second light-emitting unit. The light source is disposed underneath the abutting first and second protruding portions.