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; and wherein the second image data is recorded by an image recorder comprising: a recording head which includes a plurality of recording elements and has a two-dimensional recording element array for which a plurality of recording element columns with a plurality of recording elements arrayed in the second direction are arranged in the first direction; a moving device which relatively moves the recording head and a recording medium only once in the first direction; and a recording control device which records the second image data by the recording head while relatively moving the recording head and the recording medium, arranges dots to the individual pixels of the pixel column along the first direction by the same recording element respectively, and arranges dots to the individual pixels of the pixel column along the second direction at different timings for each recording element column.