An outline enhancing device for enhancing an outline of an image formed by a video signal having a rising and falling slope portions and a flat portion includes an enhancing signal producing circuit for producing an enhancing signal having a first, a second, and a third enhancing waveforms which wavelengths are the same as those of three portions of the video signal, respectively. A delay circuit is provided for delaying the video signal to be synchronized with the enhancing signal. A subtracter is further provided for subtracting the enhancing signal from the synchronized video signal such that three enhancing wavesforms are exactly superimposed on the rising slope, the falling slope and the flat portions, respectively, without any interference to the neighboring slopes or portion. As a result of such subtraction, the gradients of the rising and falling slopes are made large while the flat portion is extended. Thus, the outline in the video signal is enhanced without preshoots or overshoots around the outline.