In image processing it is possible to adequately reduce density unevenness and graininess according to the duty of the image data. More specifically, when dividing multi-valued data and generating 2-pass multi-pass printing data, in addition to the divided multi-valued data for each of the two passes, divided multi-valued data that is common to both of the two passes is also generated. Moreover, the quantized data of that common multi-valued data is reflected on the quantized data of each pass. Furthermore, when generating quantized data, the division ratio when generating common data in the division of multi-valued data is set according to the duty (gradation value) of the multi-valued data. By doing so it becomes possible to adequately reduce the density unevenness and graininess according to the duty of the image data.