Plural sectional images are acquired at a plurality of slice positions arranged in a predetermined direction in a subject. Each of the plural sectional images acquired by the image acquisition section is binarized based on a predetermined reference image density. Each image included in the sectional images binarized by the binarizing section is classified into a first image group having the images inside the subject and a second image group having the images outside the subject, based on the relative position of the each image from the other images in the same sectional image and also the relative position of the each image from other images contained in another sectional images.
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