To provide an X-ray thickness gauge having a calibration curve calibration function with a cost further reduced. An X-ray thickness gauge according to an embodiment measures a plate thickness of a measured object, based on a detected dose obtained by irradiating the measured object with X-ray and a calibration curve, and can execute a calibration curve calibration processing to calibrate the calibration curve. The X-ray thickness gauge stores a correction curve indicating detected dose change rates at a plurality of calibration points defining desired thicknesses, for each disturbance factor varying the detected dose. In addition, the X-ray thickness gauge stores a detected dose in a first state in which a calibration plate is not inserted in an X-ray beam, a detected dose in a second state in which the calibration plate is inserted in the X-ray beam, and a detected dose in a third state in which the X-ray is blocked, here the detected doses are used at the time of creating the calibration curve. Furthermore, the X-ray thickness gauge corrects the detected doses at the respective calibration points using the correction curve, and calibrates the calibration curve, based on the relevant corrected detected doses.
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