A color separation condition determining apparatus which, in order to produce an output image by an image forming apparatus using n color materials (22c, 22k, 22m, 22y) wherein n is an integer of 2 or greater, determines a color separation condition (SCfix) to convert first color signals representing a plurality of pixels of an input image area in m color channels wherein m is an integer of 1 or greater, into second color signals representing a plurality of pixels in n color channels that are correlated respectively with used amounts of the color materials (22c, 22k, 22m, 22y), the color separation condition determining apparatus comprising: a pixel-of-interest extractor for extracting a pixel (102, 142) of interest from the plurality of pixels represented by the first color signals input into the color separation condition determining apparatus from outside; an affecting level estimator (48, 122) for estimating affecting level of a visual effect on the pixel (102, 142) of interest in a layout of a color of the pixel (102, 142) of interest, which is extracted by the pixel-of-interest extractor, and a color of at least one of peripheral pixels (104, 144) disposed around the pixel (102, 142) of interest; a used-amount-of-color-material estimator for estimating a total used amount of the n color materials (22c, 22k, 22m, 22y) in the pixel (102, 142) of interest; and a color separation condition determiner (52, 124) for determining the color separation condition (SCfix) for the pixel (102, 142) of interest based on the affecting level of the visual effect, which is estimated by the affecting level estimator (48, 122), and the total used amount of the n color materials (22c, 22k, 22m, 22y), which is estimated by the used-amount-of-color-material estimator, wherein the affecting level estimator (48, 122) comprises a simulation image generator for generating a simulation image, which simulates the color reproduction of the output image based on the first color signals, output characteristics of the image forming apparatus and one color separation condition; and a granularity calculator for calculating a granularity (σ) based on the simulation image generated by the simulation image generator, the granularity being a psychophysical quantity and dependent on human visual response characteristics; and wherein the affecting level estimator (48, 122) is adapted to estimate the affecting level of the visual effect using the calculated granularity; and the color separation condition determiner (52, 124) includes a suitability judging section for judging whether or not a comparative color separation condition (SCc) is suitable, by comparing a standard affecting level under a standard color separation condition (SCs) and a comparative affecting level under the comparative color separation condition (SCc), the standard affecting level and the comparative affecting level being affecting levels of the visual effect, which are estimated by the affecting level estimator (48, 122) based on output characteristics of the image forming apparatus and the first color signals and each of the color separation conditions. a pixel-of-interest extractor for extracting a pixel (102, 142) of interest from the plurality of pixels represented by the first color signals input into the color separation condition determining apparatus from outside; an affecting level estimator (48, 122) for estimating affecting level of a visual effect on the pixel (102, 142) of interest in a layout of a color of the pixel (102, 142) of interest, which is extracted by the pixel-of-interest extractor, and a color of at least one of peripheral pixels (104, 144) disposed around the pixel (102, 142) of interest; a used-amount-of-color-material estimator for estimating a total used amount of the n color materials (22c, 22k, 22m, 22y) in the pixel (102, 142) of interest; and a color separation condition determiner (52, 124) for determining the color separation condition (SCfix) for the pixel (102, 142) of interest based on the affecting level of the visual effect, which is estimated by the affecting level estimator (48, 122), and the total used amount of the n color materials (22c, 22k, 22m, 22y), which is estimated by the used-amount-of-color-material estimator, wherein the affecting level estimator (48, 122) comprises a simulation image generator for generating a simulation image, which simulates the color reproduction of the output image based on the first color signals, output characteristics of the image forming apparatus and one color separation condition; and a granularity calculator for calculating a granularity (σ) based on the simulation image generated by the simulation image generator, the granularity being a psychophysical quantity and dependent on human visual response characteristics; and wherein the affecting level estimator (48, 122) is adapted to estimate the affecting level of the visual effect using the calculated granularity; and the color separation condition determiner (52, 124) includes a suitability judging section for judging whether or not a comparative color separation condition (SCc) is suitable, by comparing a standard affecting level under a standard color separation condition (SCs) and a comparative affecting level under the comparative color separation condition (SCc), the standard affecting level and the comparative affecting level being affecting levels of the visual effect, which are estimated by the affecting level estimator (48, 122) based on output characteristics of the image forming apparatus and the first color signals and each of the color separation conditions.