PURPOSE:To measure the thickness of a specimen to be inspected at a high accuracy with less number of standard pieces by such an arrangement wherein the thickness of a specimen to be inspected is calculated by using the verified curve of each range by 3-point secondary curve connection approximation to be obtained by using plural standard pieces of which thickness differs respectively. CONSTITUTION:Each of standard pieces of different thickness contained in a standard magazing 21 is inserted into a radiation passage 4 by the control of a processor 14 and the output of a detector 8 by transmitted X rays passes through an amplifier 10 and an AD converter 12 and digital output signal V is given to the processor 14. Parameters of verified curve by 3 point secondary curve connection approximation consisting of logv and thickness are memorized in a memory 18 by each range. When a specimen to be inspected 6 is inserted into the radiation passage, its thickness is calculated in the processor 14 bases on its detected output and the verifiea curve of a range corresponding to its nominal thickness and alloy compensation coefficient to be given by a set unit 16. Since verified curves by secondary curve connection are used, a high accuracy can be obtained by using less number of standard pieces.
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