A flat-panel display device using an optical waveguide includes a light source for emitting light, a plurality of optical waveguides into which light emitted from the light source is incident, an optical waveguide arrangement substrate on which the plurality of optical waveguides are arranges and made of a material having a low refractive index so as to totally reflect the light transmitted through the plurality of optical waveguides, a light output controller positioned on the plurality of optical waveguides and made of a material whose refractive index changes according to an electric field, a light output portion positioned on the light output controller, for refracting the light having passed through the light output controller when the light propagated through the plurality of optical waveguides due to the electric field is transmitted through the light output controller and is output therefrom, and outputting the same to the outside, a first electrode positioned on the light output portion, made of a transparent conductive material, and to which a predetermined control voltage is applied, a second electrode positioned under the light output controller, made of a conductive material, and which forms an electric field in conjunction with the first electrode, and a driver for applying the predetermined control voltage to the first and second electrodes. Therefore, the resolution of a reproduced image can be greatly enhanced, the light efficiency is very high and the viewing angle is increased.