Abstract: Disclosed herein is an illuminated signboard using a plane light source in which edges of two light guide plates are cut in an inclined form, an opaque plate having high surface reflectivity is inserted between the two light guide plates to optically block the two light guide plates from each other, and the light guide plates and the opaque plate are combined to form a second lighting part. U-shaped frames including LEDs are connected to construct a frame housing and the second lighting part is combined with the frame housing. Light emitted from the LEDs and transmitted in the light guide plates is dispersed by a dispersing mechanism formed on the backsides of the light guide plates using paint, print ink or engraving to transmit the light guide plates and graphics or natural color pictures formed on transparent plates attached to the outer sides of the light guide plates so as to illuminate the graphics or the natural color pictures.