The CT image processor determines the position of the liver in the body of a subject based on CT image. The PET image processor determines the position of the liver in the body of a subject based on PET image. The CT image processor calculates the displacement in the positions of the liver determined by the CT and PET images. The CT image processor extracts the contour of the liver from a CT image. The CT image processor generates a correction CT image by modifying a CT image by moving a CT image to decrease the displacement of the extracted contour in the internal area. The PET image processor performs decrease correction based on the modified CT image.
