PURPOSE: To narrow a dynamic range and to enable easy correction of data and reconstitution of an image with high accuracy by providing a mask of simple construction to the front surface of a detector if a specimen is an object having a specific shape such as a pipe or the like.

CONSTITUTION: A wedge 4 is used for a cylindrical specimen 6 and a circular cylindrical mask (a material of the quality roughly the same as the material of the wedge) 9 is provided on a radiation source 1 side in the radiation irradiating field for the position corresponding to the cavity part of the specimen 6. The mask 9 is integrally rotatively moved so as to be positioned always to the cavity part of the specimen 6 according to the rotative movement of the radiation source 1 and a detector 2 around the specimen 6. The two-dimensional radiation intensity distribution in the one projecting direction detected by each detecting element (channel) of the detector 2 is compressed and is narrowed in the dynamic range. The correction data for obtaining a CT value is easily obtainable.