PURPOSE: To obtain the stable and highly precise device in a dual-beam mode by conducting the calibration of a wedge-shaped reference piece by a reference plate which radiation beams for measurement transmits.

CONSTITUTION: At the time of calibration, an operational amplifier 36 is stopped, prescribed plate thickness is set by a plate-thickness setting apparatus 34, a reference plate 33a corresponding to the set plate thickness is inserted into the radiation beam 31 for measurement by means of a standard changer 33, the radiation beam which transmits the plate is detected by a detector 32a, and a signal for plate thickness therefrom is inputted to one input terminal of an operational amplifier 37. Then, the wedge-shaped reference piece 41 is moved by a servo mechanism 40 to a position where the thickness of the radiation beam coincides with that of the reference plate 33a, and, while the piece is fixed to the position, the reference plate 33a is removed from the inside of the radiation beam 31a for measurement and instead a substance to be measured is inserted therein. Under the condition, the signal for the plate thickness of the substance to be measured and that of the wedge-shaped reference piece 41 are compared with each other by the operational amplifier 36 and the deviation between them is indicated in an indicator 39, whereby the thickness of the substance is measured.