An image data processing device for converting original data containing gradation-value data, into processed data containing multilevel-value data. The processing device includes: (a) a processed-image gradation-value data generator for generating, based on the original data, gradation-value data relating to each pixel of the processed image, by causing a plurality of pixel lines constituting the processed image to be sequentially subjected to a line processing; (b) a processed-image multilevel-value data generator for generating, based on the generated gradation-value data and according to an error diffusion technique, the multilevel value data relating to each pixel of the processed image, by causing the plurality of pixel lines to be sequentially subjected to a line processing; and (c) a line-processing direction controller for causing the line processings to be executed on each pixel line in the same direction, and to invert each of processing directions of the respective line processings each time when the subjected pixel line is shifted from one to another. Also disclosed is an image data processing method.