1. An image processing apparatus for extracting a dark portion area from an intraluminal image, the image processing apparatus comprising: a low-luminance area detecting unit that detects a low-luminance area based on pixel values of pixels of the intraluminal image; a peripheral feature data calculating unit that calculates peripheral feature data based on pixel values of periphery of the low-luminance area; and a dark portion area determining unit that determines whether or not the low-luminance area is the dark portion area based on the peripheral feature data.