A pulse power source comprises a first circuit, a second circuit, a transformer for coupling the first circuit and the second circuit, and a switching controller. The second circuit comprises a third semiconductor switch connected in series with a secondary winding of the transformer. The third semiconductor switch is connected in such a direction that a voltage generated in the second circuit is reverse-biased during a period in which the second semiconductor switch is turned on. A gate amplifier for forming a control signal from the switching controller into a pulse and outputting the pulse as a pulse signal is connected between a gate terminal and a cathode terminal of the third semiconductor switch.