PURPOSE: To enable measurement of X-ray distribution that can be applied to a nuclear fusion reactor of practical use class and is highly accurate as well by exhausting ionized gas in each gas chamber, from a pump side ion chamber via a gas flow passage.

CONSTITUTION: A solenoid valve 18 is opened by control signal from a computer 23 as well as a pump 17 is actuated to exhaust depleted ionized gas in an ion chamber array 11. Accompanied with exhausting of the depleted ionized gas, a solenoid valve 15 is also opened. By this produced, not-depleted ionized gas in a gas cylinder 14, is supplied into the ion chamber 11 to be exchanged with another gas. Also, X-ray taken into a vacuum chamber 10 is emitted into the ion chamber array 11 via a collimeter 25. Next to this, detection signal is output from the ion chamber 11 to which the X-ray is emitted in, and, after processing through a signal processing device 22, the signal is taken in to the computer 23. In this way, measurement of spatial distribution of the X-ray can be executed highly accurately.