In a multipass printing that performs a plurality of printing scans over a unit area of a print medium by using a leading head and a follower head, the multivalued image data for the leading head is distributed according to the print volume information for up to the preceding printing scan. Then, the multivalued image data for the leading head is subjected to the grayscale level reduction operation to generate binary data. Based on the binary data, the print volume information is updated, after which the multivalued image data for the follower head is distributed according to the print volume information. This causes a plurality of printing scans or a plurality of print heads, that print the same unit area, to have a correlation among them, with the result that dots on a plurality of planes when overlapped have an excellent scattering characteristic.