An X-ray thickness gauge for measuring 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 for executing a calibration curve calibration processing to calibrate the calibration curve, the X-ray thickness gauge, comprising: first storage means for storing a plurality of correction curves each indicating detected dose change rates at a plurality of calibration points defining desired thicknesses, for each disturbance factor varying the detected dose; second storage means for storing 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, the detected doses being used at the time of creating the calibration curve; measuring means for measuring detected doses in the first to third states, respectively, at the time of calibration processing of the calibration curve; first calculation means for calculating first detected dose change rates indicating change rates of the detected doses in the first to third states, based on the respective measured detected doses and the respective stored detected doses; second calculation means for calculating a second detected dose change rate indicating a change rate of the detected doses at each of the calibration points, based on the calculated first detected dose change rates; reading means for reading a correction curve which coincides with or is approximated to a curve indicating the calculated second detected dose change rate, out of a plurality of the stored correction curves, from the first storage means; and calibration means for correcting the detected doses at the respective calibration points using the read correction curve, and for calibrating the calibration curve, based on the relevant corrected detected doses. first storage means for storing a plurality of correction curves each indicating detected dose change rates at a plurality of calibration points defining desired thicknesses, for each disturbance factor varying the detected dose; second storage means for storing 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, the detected doses being used at the time of creating the calibration curve; measuring means for measuring detected doses in the first to third states, respectively, at the time of calibration processing of the calibration curve; first calculation means for calculating first detected dose change rates indicating change rates of the detected doses in the first to third states, based on the respective measured detected doses and the respective stored detected doses; second calculation means for calculating a second detected dose change rate indicating a change rate of the detected doses at each of the calibration points, based on the calculated first detected dose change rates; reading means for reading a correction curve which coincides with or is approximated to a curve indicating the calculated second detected dose change rate, out of a plurality of the stored correction curves, from the first storage means; and calibration means for correcting the detected doses at the respective calibration points using the read correction curve, and for calibrating the calibration curve, based on the relevant corrected detected doses.