1. A radiation area extracting method for extracting a radiation area from image data, the method comprising: generating combinations of at least two candidate lines from an area of the image data, each combination of the at least two candidate lines forming a curve having a plurality of edge points; an evaluating step of obtaining evaluated values of the combinations of at least two candidate lines based upon a quantity of edge points on the curve for each combination; and an extracting step of extracting one of the combinations of at least two candidate lines as a contour line of the radiation area on the basis of the evaluated values, wherein the method is performed by a processor.