In a flash x-ray generator, a 150-nF condenser is charged up to 80 kV by a power supply, and flash x-rays are produced by the discharge. The x-ray tube is a demountable diode, and the turbomolecular pump evacuates air from the tube with a pressure of approximately 1 mPa. Since the electric circuit of the high-voltage pulse generator employs a cable transmission line, the high-voltage pulse generator produces twice the potential of the condenser charging voltage. Because bremsstrahlung rays are not emitted in the opposite direction of that of electron trajectory, clean molybdenum K-series characteristic x-rays can be produced without using a filter. When the charging voltage is increased, the K-series characteristic x-ray intensities of molybdenum increase. The K lines are clean and intense, and hardly any bremsstrahlung rays are detected. The x-ray pulse widths are approximately 100 ns, and the time-integrated x-ray intensity has a value of approximately 500 µGy per pulse at 1.0 m from the x-ray source, with a charging voltage of 80 kV.