A gas injection type plasma X-ray source has a gas plenum for storing a discharging gas at a pressure in the range of 150 Torr and 1000 Torr, the stored gas being injected between a pair of electrodes through a gas valve. The electrodes are opposed to each other in a vacuum vessel, so that a gas jet for the production of a plasma is formed. A voltage is applied between the electrodes, so that a discharge plasma is produced between said electrodes. A linear plasma with a high temperature and a high density is produced by the pinch of the plasma due to its own magnetic field produced by the current flowing through the plasma, so that X-rays are emitted from the linear plasma. The X-ray source has a high conversion efficiency and a high discharge timing margin, and accordingly the stability and reproducibility of discharges are improved and the X-ray output is increased.