PURPOSE: To provide an ionization chamber type X-ray sensor which can decrease the stress and strain in the part with an X-ray incident window.

CONSTITUTION: With a high pressure, Xe gas is encapsulated in a vessel 1 fitted interiorly with an electrode in the form of parallel flat plates. To form a concave section, a groove 6 is furnished at the outside circumferential wall 4 of vessel 1 on its X-ray incident side, and an X-ray incident window 5 is formed in the bottom surface of this groove 6. An X-ray transmissive FRP plate 10 is installed so as to cover the whole groove 6, and the part with the window 5 is reinforced. One example of the FRP plate 10 is a C-FRP which uses carbon-(C) as reinforcing fiber, wherein the fiber orientation is such as straddling the groove 6. The Xe gas in the vessel 1 is willing to push wide the outside circumferential wall 4 of the vessel 1 to the outside and to widen the groove 6, but provision of the FRP plate 10 in such a way as covering the groove 6 checks the stress of widening the groove 6, and the stress and strain in the part with the window 5 are decreased.