PURPOSE:To improve correspondence with a range and measuring accuracy, by constituting an apparatus so that the effective measuring width of a wide radiation beam can be changed in correspondence with the outer diameter of a tubular material, and obtaining the adequate value of the rate of the amount of attenuation with respect to the total amount of radiation. CONSTITUTION:Rods 26A and 26B are turned in correspondence with the diameter D of a tubular material 10. The width of radiation L is adjusted so that the radiation width L of a wide beam 14 is larger than the outer diameter D of the tubular material 10 and the rate of the amount of radiation with respect to the total amount of radiation becomes an adequate value. Then a specified reference sample is inserted in a place in which a material to be measured is entered. After the calibration of a thickness measuring scale, on-line measurement is performed. Thus, the optimum radiation width L corresponding to the outer diameter D of the tubular material 10 and the radiation density in the direction of the width can be obtained. Therefore, the total radiation amount, which is inputted to a radiation detector 30, is made approximately constant regardless of the change in the outer diameter D of the tubular material 10, and the highly accurate measurement can be performed.
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