1. A region extraction apparatus that extracts a predetermined region from image data, comprising: a processor including a region extraction unit that performs thresholding on intensity values of pixels of image data of a process target, and extracts, from the image data, a high intensity region having higher intensity than a neighboring region thereof and a low intensity region having lower intensity than a neighboring region thereof; the processor including a combination identification unit that identifies a combination of a high intensity region and a low intensity region corresponding to an extraction target region to be extracted based on arrangement positions of the high intensity region and the low intensity region extracted by the region extraction unit; the processor including an outer frame determination unit that determines an arrangement position and size of an outer frame corresponding to the combination of the high intensity region and the low intensity region based on the arrangement positions and sizes of the high intensity region and the low intensity region of the combination identified by the combination identification unit; and the processor including a contour extraction unit that extracts a contour of the extraction target region based on image data within the outer frame determined by the outer frame determination unit.