1. A method of processing an image for extracting a dark portion area, which is an area inside of a lumen which receives little light, from an intraluminal image, the method comprising: detecting a low-luminance area based on pixel values of pixels of the intraluminal image; setting a periphery area that is expanded from the low-luminance area; calculating peripheral feature data based on the pixel values of the periphery area of the low-luminance area after the periphery area of the low-luminance area is set; and determining whether or not the low-luminance area is a dark portion area based on the peripheral feature data.