In an image processing apparatus, a color image is read with pixels aligned along a direction to provide first color data on the color image. The first color data includes red, green and blue data at each pixel. A first operator calculates first chroma data according to the first color data. On the other hand, a phase shift device generates second color data by shifting a phase of red data and by shifting a phase of blue data in the first color data, and a second operator calculates second chroma data according to the second color data. A selector selects chroma data having a lower chroma, and a black edge of a character a the color image is discriminated according to the selected chroma data.