To provide an image processing device, a region extraction method, and an image processing method capable of extracting a target region based on minute variations in a concentration value that exist locally and clearly displaying the extracted target region, the image processing device extracts a blood vessel region A from an image to extract a region where a CT value is smaller than an average concentration value of the blood vessel region A as a soft plaque region B. For unextracted soft plaque, a pixel pair is set in a difference region between the region A and the region B, and for each pixel Pj between the pixel pairs, whether or not the pixel value is even smaller than a value slightly smaller than the CT value of the pixel pair is determined. Hence, a portion where a pixel value slightly varies locally is extracted as soft plaque.