To satisfactorily correct sensitivity at all times by setting a prescribed low tube current value through a tube current switching means at the time of air calibration for correcting difference in sensitivity between the respective detecting elements of a detector.

An incident X-ray is converted to an electric charge by a detector 4 arranged while facing an X-ray tube bulb 3, and an input signal is converted to a voltage value by a preamplifier 5, amplified by a programmable amplifier 5a and converted to a digital value by an A/D converter 5b. A bone salt amount is calculated from measured data from the preamplifier 5 by an image processor 6 and displayed in numerical value or image. When performing air calibration before measuring the bone salt amount of a reagent 7, a low tube current value setting part 1a is selected by a tube current switching circuit 1, and a control signal is applied to an inverter unit 2 so that a tube current can become the low tube current value. Besides, when measuring the bone salt amount, a measuring time current value setting part 1b is selected, and a control signal is applied to the inverter unit 2 so that the tube current can become a high tube current value.