1. A region detecting method based on a threshold genetic deciding method, comprising: a first step of obtaining extraction-region-identifying threshold data for a plurality of sample images including a common extraction region but having average color values different from each other in accordance with a genetic algorithm; a second step of adaptively computing the extraction-region-identifying threshold data for a purposed image in accordance with the average color values of said purposed image and a relation between the thresholds and the average color values obtained in said first step; and a third step of detecting an extraction region in accordance with the threshold data computed in said second step.