To provide the ionization chamber, which efficiently and stably detects only the amount of attenuation caused by the transmission through a material to be measured at a high speed, even when the beam alignment of X rays deviates by eliminating the dispersion of sensitivity caused by an incident position.

At the upper edge part of a container 10, a terminal 12 for outputting an ionization current I is held under the state, wherein the terminal 12 is electrically insulated from the container 10. A power supply electrode 13, which is conducted to the container 10, and a signal terminal 14, which is conducted to the terminal 12, are arranged in the inside so that the electrodes 13 and 14 are separated by gas 11 so as to surround each other. Furthermore, an X-ray filter 20 formed in a plate shape is provided at the lower edge part of the container 10, and a window 16, into which X rays 15 are applied, is provided.