To provide an ionization chamber type X-ray detector enabling elimination of a dead space.

In this detector, Xe gas is sealed in a shield vessel having an X-ray incidence window 2 formed in a part, while a signal electrode 3 and a bias electrode 4 are disposed in parallel practically and provided with a gap of a prescribed distance from the X-ray incidence window 2. Either an electron (e-) or an ion (Xe+) which are ionized by an X ray entering from the X-ray incidence window 2 is attracted electrically to the signal electrode 3 by impressing a voltage between the signal electrode 3 and the bias electrode 4 and an ionization current is detected from the signal electrode 3. In this case, the potential inside the X-ray incidence window 2 is made lower than that of the signal electrode 3.