PURPOSE: To measure accurately with removed pass-angle error and pass-level error, by obtaining projection data and plate thickness data related to an edge of a specimen and obtaining from them an inclination of the specimen by determination of an edge space position.

CONSTITUTION: This apparatus is constructed with plate thickness detecting system 10 for a thickness of the plate of a specimen using radioactive rays, image developing apparatus 20 for detecting an optical image of an edge 11a of the specimen 11, and a data arithmetic operation system 30 for correction of errors caused by the inclination of the specimen 11 using these output data. By this construction, an image passing through the edge 11a is developed by an apparatus 20 and later, the image is transmitted to an edge position converting means and here, projection data concerning the edge 11a are obtained from a relation of mutual level. On the other hand, radioactive intensity data irradiated through the specimen 11 are detected 15, and these data are transmitted to a plate thickness converting means 34 for measurement of thickness data of the specimen 11. And, after edge space position of the specimen is obtained by these data and edge position projection data, the inclination of the specimen 11 is determined, pass-angle errors, etc. are calculated and the measured plate thickness data are corrected.