1. A quantization method comprising: a quantization step of quantizing first image data, and converting the first image data to second image data that indicates a binary or multivalued quantized pattern with a smaller number of gradations than that of the first image data, for which dots are arranged by the same recording element for individual pixels of a pixel column along a first direction respectively and dots are arranged at a plurality of different timings for individual pixels of a pixel column along a second direction orthogonal to the first direction; and an optimization step of performing optimization the quantization step in at least some gradations, and reducing dispersion of a generation frequency of a dot arrangement for each relative positional relation between a pixel of interest in the case that individual pixels within the quantized pattern are successively defined as the pixel of interest and vicinity pixels that are the plurality of vicinity pixels positioned in a vicinity of the pixel of interest and include an adjacent pixel adjacent to the pixel of interest in the second direction and a next adjacent pixel successively adjacent to the adjacent pixel in the first direction, compared to the dispersion before the optimization.