PURPOSE: To obtain positively an visible image corresponding to each cross-sectional surface of an object to be tomographed, by laminating accumulation type phosphorescent sheets and X ray permeable spacers alternatively and also laminating marks distinguishing each spacer so as to make contact with each sheet.

CONSTITUTION: Accumulation type phosphorescent sheets 3aW3n and X ray permeable spacers 4 are alternatively laminated and are set so that a radiation penetrating through an object to be tomographed may irradiate along the direction of an arrow X in the figure. A distinguishing mark 5a is provided at an end of a side of the X-ray permeable spacer 4a, the side of which makes contact to the accumulation type phosphorescent sheet 3a, and also another mark 5b is provided at an end of a side of the X ray permeable spacer 4b, the side of which makes contact with the delayed phosphorescent sheet 3b. In this way, distinguishing marks are provided at each end of a surface in contact with an object sheet which the spacer 4 has to distinguish, so as to be able to distinguish all the sheets 3aW3n in a cassette. Therewith, a visible image corresponding firmly to each cross-sectional surface of the photographed object can be obtained.