A bar-like side-emitting light guide suitable for re-directing an incident light beam such that the light beam emerges from each side of the structure. The guide structure includes a light guiding bar having an upper surface and a lower surface and extending along a longitudinal direction. On a cross-sectional plane of the light guiding bar, there is a first line interval on the upper surface. A second line interval, a third line interval and a coupling part are located at the lower surface. The coupling part couples with the second line interval and the third line interval. The distance from the lower surface of the light guiding bar to a reference plane of the upper surface varies such that the thickness gradually decreases from the coupling part to the sides. In addition, a planar light source module can be constructed by arranging the foregoing light guiding bars in parallel.