A side light type spread illuminating apparatus comprises: a light conductive plate having a plurality of prisms formed on a light exit surface thereof, point light sources; and a prism sheet disposed on the light conductive plate, wherein the prisms have respective circular arc configurations with a common curvature in cross section taken along a line orthogonal to the direction the prisms extend, and are arrayed such that distances each defined between apexes of adjacent two of the prisms differ irregularly from one another within a range of 10 to 200 .mu.m, and such that overall tangential angles at respective ends of the prisms vary irregularly within a range of 40 to 70 degrees, and wherein the respective ridges of the prisms are located on a common virtual plane, whereby the prism sheet are supported evenly by all of the prisms of the light conductive plate.