PURPOSE: To emphasize a contour of a body as the max. differential value and to make it possible to extract accurately any shape of the contour of the body by setting an inner part of the contour of the body in the largest region of CT values and setting an outer part of the contour of the body in the smallest region of CT values.

CONSTITUTION: A CT image is divided into three regions of CT values by using the 1st and the 2nd threshold values. On the CT image, a contour of a body 11, contours of the lungs 12, a contour of the back bone 13, a contour of a bed 14, muscles and soft tissues, the lungs, the back bone and the bed are displayed. The contour of the body 11 and the contours of the lungs 12 are emphasized, but the difference between CT values of muscles and bodily water and a CT valve of air is larger than the difference between the CT values of muscles and bodily water and CT values of lungs, a differential value of the contour 11 of the body becomes larger than a differential value of the contours 12 of the lungs. Therefore, if by using a value between the differential value of the contour 11 of the body and the differential value of the contours 12 of the lungs as the 3rd threshold value, and a pixel having a differential value larger than the 3rd threshold value is selected, the contour 11 of the body can be accurately extracted.