The purpose of this invention is to provide an X ray thickness gauge in which using a simple configuration measurement error due to temperature drift in the amount of radiation delivered by said X ray thickness gauge after thickness calibration has been performed can be corrected even while in the process of performing a measurement. Said X ray thickness gauge (100) is provided with the following: an X ray generator (1) provided with both a cooling unit (1b) that uses a cooling medium to cool an X ray tube (1a) and a power supply unit (1c) that supplies power that is applied to said X ray tube; a detector (5) that detects delivered detection radiation that has passed through an object (4) being measured; a calibration device (2) provided with a thickness reference sample; and a computation unit (6) that references a calibration table to determine the thickness of the object being measured. This X ray thickness gauge is also provided with a temperature sensor (7) that measures the temperature of the cooling medium. The computation unit is provided in advance with a temperature correction table from which a detection radiation delivery amount corresponding to the difference between the temperature of the cooling medium during calibration and the temperature of the cooling medium during a measurement can be determined. The computation unit references said temperature correction table to determine the amount of detection radiation to deliver and references the abovementioned calibration table to determine the abovementioned thickness.
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