An ink jet printing apparatus which inhibits possible bleeding by thinning color data adjacent to black data and enables printing free from image quality degradation such as gradation skip is provided. Specifically, when only one pixel has color data and pixels located vertically and horizontally adjacent to that pixel have no color data, an isolated point pixel containing a color dot is detected only in that pixel. Then, the dots other than the isolated point are thinned-out. This makes it possible to inhibit possible bleeding at the boundary between a color area and a black area and to achieve proper printing with the appropriate gradation of the entire image maintained.