The tomographic system of the present invention comprises a means for taking tomographic images at plural tomographic positions in the axial direction of the body of the subject simultaneously, a display means for displaying the obtained plural tomographic imagers, a relative distance between tomographic images computing unit, and a control means for displaying and controlling the relative distance between tomographic images and the plural tomographic images on the afore-mentioned display means. With this, the operator can see the displayed data of the relative distances between tomographic images and the length of the treatment device instantaneously, and can therefore determine the insert position of the treatment device for the subject efficiently.