An image data generating apparatus for generating image data used for printing an image on a unit area of a printing medium by a plurality of scans of a printing head, said apparatus comprising: a divider for dividing multi-valued image data representing the image to be printed on the unit area into multi-valued image data corresponding to the respective plurality of scans; and a generator for generating respective n-valued image data corresponding to the respective plurality of scans by sequentially executing quantization processes to divided multi-valued image data obtained by said divider; characterized in that said generator executes a subsequent quantization process corresponding to a first one of the plurality of scans based on a result of a precedent quantization process which has been precedently executed corresponding to a second one of the plurality of scans different from the first scan, by (i) quantizing, in the precedent quantization process, the divided multi-valued image data corresponding to the second scan to generate n-valued image data indicative of ink-ejection for pixels in the second scan, (ii) reducing the divided multi-valued image data corresponding to the first scan for pixels having n-valued image data representing ink ejection in the second scan obtained by the precedent quantization process relative to a threshold value corresponding to said pixels in the subsequent quantization process, and (iii) quantizing, in the subsequent quantization process, the divided multi-valued image data corresponding to the first scan after said reduction to generate n-valued image data indicative of ink-ejection for pixels in the first scan. a divider for dividing multi-valued image data representing the image to be printed on the unit area into multi-valued image data corresponding to the respective plurality of scans; and a generator for generating respective n-valued image data corresponding to the respective plurality of scans by sequentially executing quantization processes to divided multi-valued image data obtained by said divider; characterized in that said generator executes a subsequent quantization process corresponding to a first one of the plurality of scans based on a result of a precedent quantization process which has been precedently executed corresponding to a second one of the plurality of scans different from the first scan, by (i) quantizing, in the precedent quantization process, the divided multi-valued image data corresponding to the second scan to generate n-valued image data indicative of ink-ejection for pixels in the second scan, (ii) reducing the divided multi-valued image data corresponding to the first scan for pixels having n-valued image data representing ink ejection in the second scan obtained by the precedent quantization process relative to a threshold value corresponding to said pixels in the subsequent quantization process, and (iii) quantizing, in the subsequent quantization process, the divided multi-valued image data corresponding to the first scan after said reduction to generate n-valued image data indicative of ink-ejection for pixels in the first scan. (i) quantizing, in the precedent quantization process, the divided multi-valued image data corresponding to the second scan to generate n-valued image data indicative of ink-ejection for pixels in the second scan, (ii) reducing the divided multi-valued image data corresponding to the first scan for pixels having n-valued image data representing ink ejection in the second scan obtained by the precedent quantization process relative to a threshold value corresponding to said pixels in the subsequent quantization process, and (iii) quantizing, in the subsequent quantization process, the divided multi-valued image data corresponding to the first scan after said reduction to generate n-valued image data indicative of ink-ejection for pixels in the first scan.