1. An image analysis apparatus comprising: an image acquisition section that acquires a plurality of sectional images at each of a plurality of slice positions arranged in a predetermined direction in a subject; a binarizing section that binarizes each of the plurality of sectional images acquired by the image acquisition section, based on a predetermined reference image density; and a classification section that classifies each image contained in the sectional image binarized by the binarizing section 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.