Provided are a data processing method and a data processor for ink jet printing, which are capable of achieving uniform and high-quality images while stabilizing density and color development in each of pixels. To this end, a mask pattern for setting permission and non-permission to print dots in each area is arranged non-periodically by using an integral multiple of m×n areas as one unit. The m×n areas allow one pixel to be expressed in half-tone. Thereby, density in the pixel is stable since a plurality of dots printed in the same pixel are printed approximately in the same event. Moreover, since each of the units (clusters) is non-periodically arranged, a uniform image can be obtained.