The invention distinguishes not only character areas and halftone dot areas in an image, but also makes highly accurate decisions concerning characters present in halftone dot areas. A decision signal generating unit generates a decision signal having luminance as a main component from a signal indicating inputted image data, and supplies the decision signal to a character decision unit, a halftone dot decision unit, and a character-in-halftone dot decision unit. The character decision unit generates data indicating whether a pixel of interest is inside a character image area. The halftone dot decision unit generates data indicating whether a pixel of interest is inside a halftone dot area. The character-in-halftone dot decision unit generates data indicating whether there is a character image inside the halftone dot area. Based on these three signals, an attribute flag generating unit generates attribute data of the pixel of interest.