A plasma X-ray source comprises inner and outer cylindrical electrodes disposed coaxially and with a certain distance with respect to each other, an electrical insulator disposed between end portions of the inner and outer cylindrical electrodes, and a discharge vessel disposed to envelop the inner and outer cylindrical electrodes. A pulse voltage is applied between the inner and outer cylindrical electrodes to produce plasma in the discharge vessel. An electrically conductive spherical shield is disposed to envelop a space where the plasma is pinched, and the spherical shield is maintained at a potential equal to that applied to the outer cylindrical electrode.