1. A region detecting method based on a method of deciding a threshold value, comprising: an input step of an input interface section inputting a plurality of sample images and a targeted image and storing the plurality of sample images and the targeted image in a storing section; a first step of a processor obtaining extraction-region-identifying threshold data for identifying an extraction region for the plurality of sample images including a common extraction region but having average color values different from each other in accordance with an algorithm; said first step including a threshold data group generating step of the processor initially generating a predetermined number of threshold data groups constituted by at least RGB threshold data for a first color-component region and RGB threshold data for a second color-component region adjacent to the first color-component region for characterizing the extraction region as the extraction-region-identifying threshold data for identifying the extraction region, an adaptability evaluating step of the processor evaluating an adaptability for the sample images of the generated threshold data groups, a gene processing step of the processor generating new threshold data groups by executing at least one of a crossing operation and a mutation operation and selecting the new threshold data groups in accordance with the adaptability evaluated in said adaptability evaluating step, a threshold data group supplying step of the processor generating additional threshold data groups to maintain a number of the threshold data groups equal to a number of the threshold data groups generated in said threshold data group generating step, and a generation alternating step of the processor repeating said gene processing step, said adaptability evaluating step and said threshold data group supplying step over set generations; a second step of the processor adaptively computing the extraction-region-identifying threshold data for identifying the extraction region for the targeted image in accordance with average color values of the targeted image and a relation between the thresholds and the average color values obtained in said first step; a third step of the processor detecting the extraction region in accordance with the threshold data computed in said second step; and an output step of the processor outputting the extraction region detected in said third step to an output interface section.