A plasma X-ray source is operable, like the plasma focus type source, to generate impulse discharge through a gas filled in a discharge tube. A relatively small amount of the gas is continuously supplied to and evacuated from the discharge tube in accordance with an occurrence frequency of discharge, without operating evacuation and filling of the gas at each cycle of discharge, under the regulation to make constant the pressure within the discharge tube, thereby eliminating the time for exchange of gas contaminated by discharge, reducing the amount of gas used, and improving reproducibility of discharge.