A plasma X-ray source in which a discharge tube containing a pair of coaxial cylindrical electrodes is filled with a gas, a high voltage pulse from a charged capacitor is applied between the cylindrical electrodes to convert the gas into a plasma, the plasma is focused on a position near the end of the inner one of the cylindrical electrodes to generate X-rays, and the X-rays thus generated are emitted to the outside of the discharge tube through a window provided thereon, is disclosed. In the above X-ray source, the high voltage pulse is applied between the cylindrical electrodes so that the inner cylindrical electrode is at a negative potential with respect to the outer cylindrical electrode.