An X-ray thickness gauge comprising: an X-ray generator which is provided with an X-ray tube a cooling unit to cool the X-ray tube with cooling medium, a power source unit to stabilize and supply a high voltage power source to be applied to the relevant X-ray tube and a filament current, and which is provided to generate X-ray; a detector to detect a detection exposure dose after an exposure dose irradiated from the X-ray generator has transmitted through a measured object; a calibration device provided with a thickness reference piece to be provided in the route of the irradiated X-ray; and a calculation unit which is arranged to obtain an attenuation amount of the X-ray by the measured object from an output of the detector, and to obtain a thickness of the measured object with reference to a calibration table that correlates a thickness of the thickness reference piece and the transmitted detection dose; the X-ray thickness gauge characterized by further comprising: a temperature sensor to measure a temperature of the cooling medium; wherein the calculation unit is provided with a temperature correction table or a temperature correction expression which obtains the detection exposure dose corresponding to a temperature difference between a temperature (Tr) of the cooling medium measured using the thickness reference piece at the time of calibration, and a temperature (Tm) of the cooling medium during measurement to measure the measured object; the calculation unit being arranged to obtain the difference (Tm-Tr) between the temperature of the cooling medium during the measurement and the temperature at the time of the calibration, and to obtain the corresponding detection exposure dose with reference to the temperature correction table or the temperature correction expression; and further to obtain the thickness with reference to the calibration table; and thereby correction of a thickness measurement error is enabled even during the measurement. an X-ray generator which is provided with an X-ray tube a cooling unit to cool the X-ray tube with cooling medium, a power source unit to stabilize and supply a high voltage power source to be applied to the relevant X-ray tube and a filament current, and which is provided to generate X-ray; a detector to detect a detection exposure dose after an exposure dose irradiated from the X-ray generator has transmitted through a measured object; a calibration device provided with a thickness reference piece to be provided in the route of the irradiated X-ray; and a calculation unit which is arranged to obtain an attenuation amount of the X-ray by the measured object from an output of the detector, and to obtain a thickness of the measured object with reference to a calibration table that correlates a thickness of the thickness reference piece and the transmitted detection dose; the X-ray thickness gauge characterized by further comprising: a temperature sensor to measure a temperature of the cooling medium; wherein the calculation unit is provided with a temperature correction table or a temperature correction expression which obtains the detection exposure dose corresponding to a temperature difference between a temperature (Tr) of the cooling medium measured using the thickness reference piece at the time of calibration, and a temperature (Tm) of the cooling medium during measurement to measure the measured object; the calculation unit being arranged to obtain the difference (Tm-Tr) between the temperature of the cooling medium during the measurement and the temperature at the time of the calibration, and to obtain the corresponding detection exposure dose with reference to the temperature correction table or the temperature correction expression; and further to obtain the thickness with reference to the calibration table; and thereby correction of a thickness measurement error is enabled even during the measurement. a temperature sensor to measure a temperature of the cooling medium; wherein the calculation unit is provided with a temperature correction table or a temperature correction expression which obtains the detection exposure dose corresponding to a temperature difference between a temperature (Tr) of the cooling medium measured using the thickness reference piece at the time of calibration, and a temperature (Tm) of the cooling medium during measurement to measure the measured object; the calculation unit being arranged to obtain the difference (Tm-Tr) between the temperature of the cooling medium during the measurement and the temperature at the time of the calibration, and to obtain the corresponding detection exposure dose with reference to the temperature correction table or the temperature correction expression; and further to obtain the thickness with reference to the calibration table; and thereby correction of a thickness measurement error is enabled even during the measurement.